gdbserver: add back lost comments in fast_tracepoint_ctx

gdbserver: add back lost comments in fast_tracepoint_ctx

Before the removal of the UST/static-tracepoint support, the
`static_tracepoint_ctx` struct contained comments for its fields,
whereas `fast_tracepoint_ctx` did not.  Nevertheless, those comments
also applied to `fast_tracepoint_ctx`.  With the removal of
`static_tracepoint_ctx`, the comments were lost, making
`fast_tracepoint_ctx` data members completely commentless.  Add back
those comments.

Approved-By: Simon Marchi <simon.marchi@efficios.com>