Misc docfixes.

Misc docfixes.
Suggested by Martin Rudalics.

* doc/lispref/display.texi (Face Functions): Fix define-obsolete-face-alias.

* doc/lispref/functions.texi (Obsolete Functions): Fix doc for
set-advertised-calling-convention.

* doc/lispref/modes.texi (Mode Help): Fix describe-mode.

* doc/lispref/variables.texi (Variable Aliases): Fix make-obsolete-variable.

* lisp/help.el (describe-mode): Doc fix.