Minor updates to Markers and Text chapters of Lisp manual.

Minor updates to Markers and Text chapters of Lisp manual.

* doc/lispref/markers.texi (The Mark): Fix typo.
(The Region): Copyedits.

* doc/lispref/text.texi (Buffer Contents): Don't duplicate explanation of
region arguments from Text node.  Put doc of obsolete var
buffer-substring-filters back, since it is referred to.
(Low-Level Kill Ring): Yank now uses clipboard instead of primary
selection by default.