zmacs: show buffer-local details in describe-variable output.

* lib/zmacs/zlisp/help.zl (write_variable_description): If this
is a buffer-local variable, output a notice to that effect.  If
it has a different local value to the original default, also show
the original value.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>