document segment selector and base information vs mcontext

We should add this info to the docs, from an email sent in reply to a question on the users list:

If you mean the segment selector value in the register, it should work to
call dr_switch_to_app_state() and then read the current register. If you
mean the segment base it points to, see dr_insert_get_seg_base(),
dr_get_dr_segment_base(), or call opnd_compute_address() on fs:0 or sthg.