Update explain_fft.rst for cleaner layout (#8100)

As others, this PR places directives and modifiers on lines with not spaces between (like the master version).  Used in grdfft and potential's gravfft and grdflexure.

doc/rst/source/explain_fft.rst_

@@ -1,56 +1,50 @@
 **-N**\ [**a**\|\ **f**\|\ **m**\|\ **r**\|\ **s**\|\ *nx/ny*][**+a**\|\ **d**\|\ **h**\|\ **l**][**+e**\|\ **n**\|\ **m**][**+t**\ *width*][**+v**][**+w**\ [*suffix*]][**+z**\ [**p**]]
     Choose or inquire about suitable grid dimensions for FFT and set
-    optional parameters. Control the FFT dimension:
+    optional parameters. Control the FFT dimension via these directives:
 
-    	**-Na** lets the FFT select dimensions yielding the most accurate result.
+    - **a**: Let the FFT select dimensions yielding the most accurate result.
+    - **f**: Force the FFT to use the actual dimensions of the data.
+    - **m**: Let the FFT select dimensions using the least work memory.
+    - **r**: Let the FFT select dimensions yielding the most rapid calculation.
+    - **s**: Just present a list of optional dimensions, then exit.
 
-        **-Nf** will force the FFT to use the actual dimensions of the data.
+    Without a directive we expect **-N**\ *nx/ny* which will do FFT on array size *nx/ny*
+    (must be >= grid file size). Default chooses dimensions >= data which optimize speed
+    and accuracy of FFT. If FFT dimensions > grid file dimensions, data are extended and
+    tapered to zero.
 
-    	**-Nm** lets the FFT select dimensions using the least work memory.
+    Control detrending of data by appending a modifier for removing a linear trend.
+    Consult module documentation for the default action:
 
-    	**-Nr** lets the FFT select dimensions yielding the most rapid calculation.
+    - **+d**: Detrend data, i.e. remove best-fitting linear trend.
+    - **+a**: Only remove the mean value.
+    - **+h**: Only remove the mid value, i.e. 0.5 * (max + min).
+    - **+l**: Leave data alone.
 
-        **-Ns** will present a list of optional dimensions, then exit.
+    Control extension and tapering of data by appending a modifier to control how
+    the extension and tapering are to be performed:
 
-        **-N**\ *nx/ny* will do FFT on array size *nx/ny* (must be >= grid file size). Default chooses
-        dimensions >= data which optimize speed and accuracy of FFT. If FFT
-        dimensions > grid file dimensions, data are extended and tapered to zero.
+    - **+e**: Extend the grid by imposing edge-point symmetry [Default].
+    - **+m**: Extends the grid by imposing edge mirror symmetry.
+    - **+n**: Turns off data extension.
 
-    Control detrending of data: Append modifiers for removing a linear trend. Consult module documentation for the default action:
+    Tapering is performed from the data edge to the FFT grid edge [100%].
+    Change this percentage via modifier **+t**\ *width*. When **+n** is in effect,
+    the tapering is applied instead to the data margins as no extension is
+    available [0%].
 
-        **+d**: Detrend data, i.e. remove best-fitting linear trend.
+    Control messages being reported:
 
-        **+a**: Only remove mean value.
-
-        **+h**: Only remove mid value, i.e. 0.5 * (max + min).
-
-        **+l**: Leave data alone.
-
-    Control extension and tapering of data:
-    Use modifiers to control how the extension and tapering are to be performed:
-
-        **+e** extends the grid by imposing edge-point symmetry [Default].
-
-        **+m** extends the grid by imposing edge mirror symmetry.
-
-        **+n** turns off data extension.
-
-        Tapering is performed from the data edge to the FFT grid edge [100%].
-        Change this percentage via **+t**\ *width*. When **+n** is in effect,
-        the tapering is applied instead to the data margins as no extension is
-        available [0%].
-
-	Control messages being reported:
-        **+v** will report suitable dimensions during processing.
+    - **+v**: Report suitable dimensions during processing.
 
     Control writing of temporary results:
     For detailed investigation you can write the intermediate grid being passed
     to the forward FFT; this is likely to have been detrended, extended by
-    point-symmetry along all edges, and tapered. Append **+w**\ [*suffix*] from
-    which output file name(s) will be created (i.e., *ingrid_prefix.ext*)
-    [tapered], where *ext* is your file extension. Finally, you may save
-    the complex grid produced by the forward FFT by appending **+z**. By
-    default we write the real and imaginary components to
-    *ingrid*\ \_real.\ *ext* and *ingrid*\ \_imag.\ *ext*. Append
-    **p** to save instead the polar form of magnitude and phase to files
-    *ingrid*\ \_mag.\ *ext* and *ingrid*\ \_phase.\ *ext*.
+    point-symmetry along all edges, and tapered. Use these modifiers to ave such grids:
+
+    - **+w**: Set the *suffix* from which output file name(s) will be created
+      (i.e., *ingrid_prefix.ext*) [Default is "tapered"], where *ext* is your file extension
+    - **+z**:  Save the complex grid produced by the forward FFT. By default we
+      write the real and imaginary components to *ingrid*\ \_real.\ *ext* and
+      *ingrid*\ \_imag.\ *ext*.  Append **p** to instead use the polar form of magnitude
+      and phase to files *ingrid*\ \_mag.\ *ext* and *ingrid*\ \_phase.\ *ext*.