Document the -catch-assert and -catch-exception new GDB/MI commands.

Document the -catch-assert and -catch-exception new GDB/MI commands.

This patch adds documentation for the new GDB/MI commands "-catch-assert"
and "-catch-exception", meant to provide the same functionality as
the "catch assert", "catch exception" and "catch exception unhandled"
CLI commands.

In the GDB Manual, there was already a section for catchpoint comments,
so that seemed like a natural place to document the new commands. But
commands related to a given concept seem to have traditionally been
organized alphabetically, and I didn't want future commands to break
down logical pairing of various commands. For instance, "-catch-load"
and "-catch-unload" are quite "distant" from each other, and it is easy
to imagine a new comment which would alphabetically fall in between,
causing them to be separated. So I introduced subsections to prevent
that from happening.

gdb/ChangeLog:

        * NEWS: Add entry documenting the new "-catch-assert" and
        "-catch-exception" GDB/MI commands.

gdb/doc/ChangeLog:

        * gdb.texinfo (Shared Library GDB/MI Catchpoint Commands):
        New subsection inside which the "-catch-load" and "-catch-unload"
        commands documentation is now placed.
        (Ada Exception GDB/MI Catchpoint Commands): New subsection
        documenting the "-catch-assert" and "-catch-exception" new
        GDB/MI commands.