clarify docs

REQUIRE

@@ -1 +1 @@
-julia 0.7-beta
+julia 0.7-beta2

src/TreeViews.jl

@@ -22,6 +22,8 @@ numberofnodes(x::T) where {T} = fieldcount(T)
     treelabel(io::IO, x, mime = MIME"text/plain"())
 
 Prints `x`'s tree header to `io`.
+Like with `Base.show` there are also methods with `mime::AbstractString` and no `mime` argument at all (which
+falls back to `MIME"text/plain"()`). Please only overload the `treelabel(io::IO, x, mime::MIME)` form.
 """
 treelabel(io::IO, x::T, mime::MIME"text/plain") where {T} = show(io, mime, T)
 treelabel(io::IO, x::T, mime::AbstractString)  where {T} = treelabel(io, x, MIME(mime))
@@ -31,6 +33,8 @@ treelabel(io::IO, x::T)  where {T} = treelabel(io, x, MIME"text/plain"())
     nodelabel(io::IO, x::T, i::Integer, mime::MIME"text/plain" = MIME"text/plain"())
 
 Prints the label of `x`'s `i`-th child to `io`.
+Like with `Base.show` there are also methods with `mime::AbstractString` and no `mime` argument at all (which
+falls back to `MIME"text/plain"()`). Please only overload the `nodelabel(io::IO, x, i::Integer, mime::MIME)` form.
 """
 function nodelabel(io::IO, x::T, i::Integer, mime::MIME"text/plain") where {T}
   show(io, mime, Text(String(fieldname(T, i))))