* lisp/subr.el (decode-char, encode-char): Use advertised-calling-convention

* lisp/subr.el (decode-char, encode-char): Use advertised-calling-convention
instead of the docstring to discourage use of the `restriction' arg.
(error): Use `declare'.
* src/charset.c (Fdecode_char, Fencode_char): Remove description of
`restriction' arg. now that it's hidden by advertised-calling-convention.