Allow leading doc-strings when (re-)defining faces via :custom-face

[antler5]

Requires such doc-strings to be typed as literal strings at compile-time -- no (concat [...]) forms.

Just a piece of my personal config for postarity.

Includes updates to the README, manual, and tests, though I left it unspoken that the string must be a literal.

I like #1048 too, I think that checks out.