gdb/manual: Move @findex entries

gdb/manual: Move @findex entries

The manual currently has many cases like these:

 @item $_gdb_setting_str (@var{setting})
 @findex $_gdb_setting_str@r{, convenience function}

As suggested by Eli, move the @findex entries before @item so that the
index records the position of @item, and the Info reader places you
there when you use index-search.

I went over all @findex calls in the manual, and most are like the
above.  Most either appear before @item, or before @subheading, like:

 @subheading The @code{-break-after} Command
 @findex -break-after

I fixed all of them.

There are findex entries in annotate.texinfo,python.texi, and
stabs.texinfo as well, though those all look right to me already.

Tested by typing "i _isvoid" (@item case) and "i -complete"
(@subheading case) in an Info reader, and checking where those took
me.

Change-Id: Idb6903b0bb39ff03f93524628dcef86b5585c97e
Suggested-By: Eli Zaretskii <eliz@gnu.org>