doc: Include the contents of ABOUT-NLS in the documentation.
* gettext-runtime/doc/nls.texi (The original ABOUT-NLS): Renamed from
'Translation Intro'. Turn into a section. Mark section as historical.
* gettext-tools/doc/gettext.texi: Include nls.texi.
* gettext-tools/doc/Makefile.am (gettext_TEXINFOS): Add nls.texi and
matrix.texi.
