Document completion styles in Emacs manual; copyedits.

Document completion styles in Emacs manual; copyedits.

* doc/emacs/mini.texi (Minibuffer File): Minor copyedits.  Use xref to
Remote Files node instead of linking directly to the Tramp manual.
(Minibuffer Edit): Add xref to Blank Lines.
(Completion): Add xref to Symbol Completion.  Remove redundant
example, which is repeated in the next node.
(Completion Commands): Minor clarifications.
(Completion Styles): New node, split from Completion Commands.
Document substring and initials styles.
(Strict Completion): Remove information duplicated in other nodes.
(Completion Options): Consolidate case difference discussion here.

* doc/emacs/files.texi (File Names): Add index entries.

* doc/emacs/help.texi (Help Mode): Fix kindex entries.