Clarify nature of "free-standing-units" (closes #750)

siunitx.tex

@@ -2456,11 +2456,15 @@ \subsection{Creating units%
 
 \DescribeOption{free-standing-units}
 \DescribeOption{overwrite-functions}
-The \opt{free-standing-units} option controls whether the unit macros exist
-outside of the \cs{unit} and \cs{qty} arguments. When this option is
+The \opt{free-standing-units} option controls whether the unit macros are
+usable outside of the \cs{unit} and \cs{qty} arguments. When this option is
 \opt{true}, \pkg{siunitx} creates the macros for general use. The standard
 method to achieve this does not overwrite any existing macros: this behaviour
-can be altered using the \opt{overwrite-commands} switch.
+can be altered using the \opt{overwrite-commands} switch. For technical
+reasons, when \opt{free-standing-units} is set \opt{false}, the names of unit
+macros still have to be defined: as such, they will be created if they do not
+exist, but will raise an error if used outside of the \cs{unit} and \cs{qty}
+arguments.
 
 When using the approach of \enquote{free-standing} unit commands, only macros
 created using \cs{DeclareSIUnit} are defined generally. Thus prefixes and