Document "info exceptions" and "-info-ada-exception" new commands.

Document "info exceptions" and "-info-ada-exception" new commands.

gdb/doc/ChangeLog:

        * gdb.texinfo (Ada): Add entry in menu for new "Ada Exceptions" node.
        (Ada Exceptions): New node.
        (GDB/MI): Add entry in menu for new "GDB/MI Ada Exceptions
        Commands" node.
        (GDB/MI Ada Exceptions Commands): New node.
        (GDB/MI Miscellaneous Commands): Document new "info-ada-exceptions"
        field in the output of the "-list-features" command.
        * NEWS: Add entry for the new "info exceptions" CLI command,
        and for the new "-info-ada-exceptions" GDB/MI command.