More doc updates.

More doc updates.

* backups.texi (Making Backups): Document backup-buffer change.

* commands.texi (Defining Commands): Document the interactive-form
property more carefully.  Document interactive-only.

* compile.texi (Compiler Errors): Copyedits.  Note that the
details for byte-compile-warnings are in its docstring.

* customize.texi (Variable Definitions): Likewise.

* files.texi (Visiting Files): Copyedits.
(Testing Accessibility): Mention ACLs.  Move file-modes here from
File Attributes.
(Truenames): Move file-equal-p here from Kinds of Files.
(File Attributes): Move file-newer-than-file-p here from Testing
Accessibility.
(Extended Attributes): New node.  Add file-extended-attributes.
(Changing Files): Document set-file-extended-attributes.

* minibuf.texi (Minibuffer Contents): Remove obsolete function
minibuffer-completion-contents.

* variables.texi (Defining Variables): Note that defvar acts
always on the dynamic value.