Documentation and commentary improvements

Documentation and commentary improvements

* src/lisp.h:
* src/regex.c:
* src/xgselect.c (xg_select): Improve commentary and formatting.

* doc/lispref/objects.texi (Thread Type, Mutex Type)
(Condition Variable Type): New subsections.
(Type Predicates): Add thread-related predicates.
* doc/lispref/objects.texi (Editing Types):
* doc/lispref/elisp.texi (Top): Update higher-level menus.