Reorganize confirm-nonexistent-file-or-buffer documentation in Emacs manual.

Reorganize confirm-nonexistent-file-or-buffer documentation in Emacs manual.

* doc/emacs/buffers.texi (Buffers): Tweak mention of mail buffer name.
(Select Buffer): Move confirmation discussion to Minibuffer Exit.

* doc/emacs/files.texi (File Names, Visiting): Move detailed discussion of
minibuffer confirmation to Minibuffer Exit.

* doc/emacs/mini.texi (Minibuffer Exit): Rename from Strict Completion.
Move confirm-nonexistent-file-or-buffer discussion here.