[gdb/doc]: Updated manpages to be consistent with help

Updated manpages to be consistent with help information provided by the
binary. The main changes are:

* Making all long-form options have '--', instead of a single '-';
* added most of the missing options to the manpage;
* removed the information about using '+' instead of '-', since it
  doesn't seem to be supported anymore.

This also fixes 2 upstream bugs:
* https://sourceware.org/bugzilla/show_bug.cgi?id=23965; by adding
--args to the manpage
* https://sourceware.org/bugzilla/show_bug.cgi?id=10619; by adding the
double dashes