(Defining Variables): Second argument to `defconst' is not optional.

(Defining Variables): Second argument to `defconst' is not optional.
(Setting Variables): Mention optional argument APPEND to `add-to-list'.
(Creating Buffer-Local): Expand description of `make-variable-buffer-local'.
(Frame-Local Variables): Expand description of `make-variable-frame-local'.
(Variable Aliases): Correct description of optional argument DOCSTRING
to `defvaralias'.  Mention return value of `defvaralias'.
(File Local Variables): Add xref to `File variables' in Emacs Manual.
Correct description of `hack-local-variables'.  Mention
`safe-local-variable' property.  Mention optional second argument to
`risky-local-variable-p'.