Fix qRcmd error code parsing

Fix qRcmd error code parsing

Someone at IRC spotted a bug in qRcmd handling. This looks like an oversight
or it is that way for historical reasons.

The code in gdb/remote.c:remote_target::rcmd uses isdigit instead of
isxdigit. One could argue that we are expecting decimal numbers, but further
below we use fromhex ().

Update the function to use isxdigit instead and also update the documentation.

I see there are lots of other cases of undocumented number format for error
messages, mostly described as NN instead of nn. For now I'll just update
this particular function.