gdb, doc: mention the 'S' option for the QTDP packet

gdb, doc: mention the 'S' option for the QTDP packet

I noticed that gdbserver accepts an 'S' option for the QTDP packet to
create a static tracepoint, but this is not mentioned in the document.
Update the document.

I first thought about updating the argument as `[:Flen|:S]`, but then
opted for `[:Flen][:S]`.  Although it is odd that ':F' and ':S' are
allowed to co-exist, the implementation at the gdbserver side allows
this and handles the packet arguments so that the right-most
positioned ':F' or ':S' overwrites the final tracepoint type.  When
the documentation is missing, the implementation usually determines
the behavior.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>