Fix problems resulting from modification of the undo list

Fix problems resulting from modification of the undo list

* doc/lispref/text.texi (Atomic Changes): Describe what not to
do inside an atomic change group.
* lisp/elec-pair.el (electric-pair-inhibit-if-helps-balance):
Don't call `delete-char'; that edits the undo list by removing
boundary markers.
* lisp/subr.el (atomic-change-group, prepare-change-group): Warn
against modifying the undo list inside.