More small edits for doc/lispref/minibuf.texi

More small edits for doc/lispref/minibuf.texi

* doc/lispref/elisp.texi, doc/lispref/vol1.texi, doc/lispref/vol2.texi:
* doc/lispref/minibuf.texi (Completion):
Update "High-Level Completion" description.

* doc/lispref/minibuf.texi (Basic Completion):
No need to describe obarrays here.
Don't mention obsolete `nospace' argument of all-completions.
(Minibuffer Completion, Completion Commands, Reading File Names)
(Completion Variables): Copyedits.
(Completion Commands): Mention parent keymaps.
Remove obsolete minibuffer-local-filename-must-match-map.
(High-Level Completion): Remove read-variable's almost
word-for-word duplication of read-command.