Document wrapper changes.

Document wrapper changes.

* NEWS: Indicate new feature and incompatibility.
* doc/libtool.texi [detailmenu]: Add reference to subsection
'Wrapper executables for programs', in Platform Quirks category.
[Linking executables]: Mention wrapper executables, in
addition to wrapper scripts. Add menu referencing subsection
'Wrapper executables for programs'.
[Wrapper executables for programs]: New subsection. Documents
cwrapper rationale and command line options.