Document that {en,de}code-coding-string preserve match data

Document that {en,de}code-coding-string preserve match data

* lisp/international/mule.el (define-coding-system):
Require :pre-write-conversion and :post-read-conversion functions
to leave the match data untouched.
* src/coding.c (Fdecode_coding_string, Fencode_coding_string):
Document functions as match-data-preserving.

Suggested by Stefan Monnier (see bug#41445).