Replace "info-ada-exceptions" by "ada-exceptions" in -list-features

Replace "info-ada-exceptions" by "ada-exceptions" in -list-features

Rather than having -list-features report support for the GDB/MI
commands providing access to Ada exception catchpoints with one entry,
and the GDB/MI command providing the list of Ada exceptions with
a second entry, this patch merges it all within one single entry.
This is OK, because all these commands were added within a short
amount of time, and within the same release cycle; and it reduces
a bit the size of the output.

gdb/ChangeLog:

        * mi/mi-main.c (mi_cmd_list_features): Replace "info-ada-exceptions"
        entry with "ada-exceptions".

gdb/doc/ChangeLog:

        * gdb.texinfo (GDB/MI Miscellaneous Commands): Delete
        the documentation of "info-ada-exceptions" in the output
        of the "-list-features" command.  Add the documentation
        of the "ada-exception" entry instead.