Initial documentation for xwidgets

[Eli Zaretskii]

I've pushed commit ef760b8, which is the initial attempt to document
the xwidget features.  The user manual mentions a single command and
the mode it enters.  The section in the ELisp manual mentions some of
the primitives I thought were important to document.  The particular
selection I decided to document, and the ones I decided to omit might
not make sense, which is OK -- the whole issue makes very little sense
to me for now, and the (abysmally inadequate, IMO) GTK documentation
of the APIs used by xwidget.c doesn't help at all.  The result cannot
be good enough by any measure.  Still, IMO it's better than no
documentation at all, and at least a seasonal WebKit programmer should
be able to figure the stuff out.

Please review and comment on functions/commands that should be added
to the documentation, and also on the correctness of the text.

Thanks.