gdb/doc: document how filename arguments are formatted

In the following commits I intend to improve GDB's filename
completion.  However, how filenames should be completed is a little
complex because GDB is not consistent with how it expects filename
arguments to be formatted.

This commit documents the current state of GDB when it comes to
formatting filename arguments.

Currently GDB will not correctly complete filenames inline with this
documentation; GDB will either fail to complete, or complete
incorrectly (i.e. the result of completion will not then be accepted
by GDB).  However, later commits in this series will fix completion.

Approved-By: Eli Zaretskii <eliz@gnu.org>