Move read-char-from-minibuffer from simple.el to subr.el and document it.

Move read-char-from-minibuffer from simple.el to subr.el and document it.

* doc/lispref/minibuf.texi (Multiple Queries):
Document read-char-from-minibuffer (bug#10477, bug#38076).

* lisp/subr.el (read-char-history)
(read-char-from-minibuffer-map)
(read-char-from-minibuffer-map-hash)
(read-char-from-minibuffer-insert-char)
(read-char-from-minibuffer-insert-other, empty-history)
(read-char-from-minibuffer): Move from simple.el to subr.el.