doc/texi2pod: make references bold

Inspired by GCC r86635.

This is more consistent with other man pages. For example in `man git`,
all the "git-help(1)" kind of cross refs are bold.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>