Shell: Document two more approaches.

Shell: Document two more approaches.

* gettext-tools/doc/lang-sh.texi (sh): Tweaks.
(sh - Three approaches): New subsubsection.
(The gettext.sh approach): New subsubsection, incorporating the "gettext.sh"
subsubsection.
(The printf approach, The printf_gettext approach): New subsubsections.
(Preparing for gettext.sh): Renamed from "Preparing Shell Scripts".
(Preparing for printf, Preparing for printf_gettext): New subsubsections.
* gettext-tools/doc/gettext.texi: Update detailed node list.
* NEWS: Mention the change.