suggestion: move "Gnulib module: --" blurb to misc notes

[Thien-Thi Nguyen]

Presently, the blurb:

|The notation ``Gnulib module: ---'' means that Gnulib does not provide a
|module providing a substitute for the function.  When the list
|``Portability problems not fixed by Gnulib'' is empty, such a module is
|not needed: No portability problems are known.  Otherwise, it indicates
|that such a module would be useful but is not available: No one so far
|found this function important enough to contribute a substitute for it.
|If you need this particular function, you may write to
|@code{<bug-gnulib at gnu dot org>}.

is in @node "Function Substitutes", which comes after usage of that notation
in the preceding @node "Header File Substitutes".  I think it would good to
move it to its own section under "Miscellaneous Notes" and then @xref to it.

I can provide a patch if there is interest.

thi