Copyedits for Internals and OS chapters of Lisp manual.

Copyedits for Internals and OS chapters of Lisp manual.

* doc/lispref/internals.texi (Building Emacs, Garbage Collection): Copyedits.
(Writing Emacs Primitives): Re-organize discussion of functions
with variable Lisp arguments are handled.  Delete an obsolete
remark, previously tagged as FIXME.

* doc/lispref/os.texi (Idle Timers): Minor clarification.
(Idle Timers): Link to Time of Day for description of time list.