* libguile/boolean.c (scm_nil_p): Improve docstring.
* doc/ref/api-languages.texi (Nil): Add documentation for 'nil?', along
with a description of how Elisp interprets Scheme booleans and
end-of-list.
* doc/ref/api-languages.texi: Move here from api-translation. Still
stubbed in parts, but it's getting better.
* doc/ref/Makefile.am: Adapt to api-languages.texi name change.
* doc/ref/guile.texi: Likewise.