Fix handling of minibuffer-only child frames (Bug#33498)

Fix handling of minibuffer-only child frames (Bug#33498)

* doc/lispref/frames.texi (Buffer Parameters): Describe how to
make a minibuffer-only child frame.
(Child Frames): Describe how minbuffer child frames are
deleted.
* src/frame.c (delete_frame): Handle deletion of minibuffer
child frames (Bug#33498).  In the course, fix reassigning of
'default-minibuffer-frame' with minibuffer-only frames.
* lisp/frame.el (frame-notice-user-settings): Handle creation of
initial minibuffer-only child frame.
(make-frame): Handle creation of frame with a minibuffer-only
child frame.