lisp/international/mule.el (define-coding-system): Improve docstring.

Backported from trunk; do not merge back.

(cherry picked from commit 5979af512bf5d6a28acea24299b67c7939d9e703)
lisp/international/mule.el (define-coding-system): Fix typos in the doc string.
Add missing ChangeLog entry for the previous commit.

Backported from trunk; do not merge back.

(cherry picked from commit 79e2dade762491c58aa6396e35bae0ef418bafc6)