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)
* build-aux/git-hooks/commit-msg: Allow tabs.

Treat them as if they were expanded to spaces, with tab stops
every 8 columns.
Merge from origin/emacs-24

The following commits were skipped:

39eaef9 lisp/international/mule.el (define-coding-system): Fix typos in the doc string. Add missing ChangeLog entry for the previous commit.
df932d8 lisp/international/mule.el (define-coding-system): Improve docstring.
Merge from origin/emacs-24

f646cd9 * build-aux/git-hooks/commit-msg: Allow tabs.