common/ptid.h: Mention that process_stratum targets should prefer ptid.lwp.

common/ptid.h: Mention that process_stratum targets should prefer ptid.lwp.

It's best that we standardize on process_stratum targets using the
ptid.lwp field to store thread ids.  The idea being leave the ptid.tid
field free for any thread_stratum target that might want to sit on
top.  This patch adds a comment in that direction to struct ptid's
definition.

gdb/
2014-02-19  Pedro Alves  <palves@redhat.com>

	* common/ptid.h (struct ptid): Mention that process_stratum
	targets should prefer ptid.lwp.