Clean up ptid.h/ptid.c.

The ptid_t contructors, accessors and predicates are documented in
_three_ places, and each place uses a different wording.

E.g, the descriptions in the .c file of the new ptid_lwp_p, ptid_tid_p
weren't updated in the final revision like the descriptions in the .h
file were.  Clearly, switching to a style that has a single central
description avoids such issues.

Worse, some of the existing descriptions are plain wrong, such as:

  /* Attempt to find and return an existing ptid with the given PID, LWP,
     and TID components.  If none exists, create a new one and return
     that.  */
  ptid_t ptid_build (int pid, long lwp, long tid);

The function does nothing that complicated.  It's just a simple
constructor.

So this gets rid of all the unnecessary descriptions, leaving only the
ones near the function declarations in the header file, and
fixes/clarifies those that remain.

gdb/
2013-10-04  Pedro Alves  <palves@redhat.com>

	* common/ptid.c (null_ptid, minus_one_ptid, ptid_build)
	(pid_to_ptid, ptid_get_pid, ptid_get_lwp, ptid_get_tid)
	(ptid_equal, ptid_is_pid, ptid_lwp_p, ptid_tid_p): Replace
	describing comments with references to ptid.h.
	* common/ptid.h: Remove intro description of constructors,
	accessors and predicates.
	(struct ptid): Reformat.
	(minus_one_ptid, ptid_build, pid_to_ptid, ptid_get_pid)
	(ptid_get_lwp, ptid_get_tid, ptid_equal, ptid_is_pid): Change
	describing comments.