gdb/doc: extend the documentation for the 'handle' command

gdb/doc: extend the documentation for the 'handle' command

The documentation for the 'handle' command does not cover all of the
features of the command, and in one case, is just wrong.

The user can specify 'all' as signal name, the documentation implies
that this will change the behaviour of all signals, in reality, this
changes all signals except SIGINT and SIGTRAP (the signals used by
GDB).  I've updated the docs to list this limitation.

The 'handle' command also allows the user to specify multiple signals
for a single command, e.g. 'handle SIGFPE SIGILL nostop pass print',
however the documentation doesn't describe this, so I've updated the
docs to describe this feature.