completing-read-multiple and read-face-name are not in Elisp manual

[Drew Adams]

(Yes, I realize that not every function needs to be documented in the
manual - please don't bother with such a general reminder.) I think
`completing-read-multiple' and `read-face-name' should be documented.

Is `completing-read-multiple' not in the manual because `crm.el' is somehow
considered optional? `completing-read-multiple' is used in `read-face-name'
(where there is also a `require' of crm).

The doc string of `read-face-name' says nothing about the fact that you can
input and complete multiple face names. And nothing indicates to a user,
when `read-face-name' is called, that s?he can input and complete multiple
names, separating them with a comma. (Sure, a programmer can mention that in
the prompt, but s?he might not, especially since there is no info about this
in the doc string.) The doc string of read-face-name mentions the optional
MULTIPLE argument, but that is only for output, not input.

I find nothing about this behavior described anywhere in the Emacs manual or
the Elisp manual. How will users understand the UI? How will programmers
understand how these functions work (without examining the code in detail)?