[doc] Update gdb-add-index manpage

[doc] Update gdb-add-index manpage

The current gdb-add-index manual page is a bit out-of-date.  This
commit fixes a few deficiencies:

- gdb-add-index does not use objdump; it uses objcopy and readelf
- missing info on environment variables (in appropriate ENVIRONMENT section).
- missing mention of -dwarf-5 option
- adds important notice about FILENAME being writable
- explains exit status
- the script adds appropriate section(s) to the file; it does not
  output new files with the section(s)

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