Some changes in comments and documentation.

Some changes in comments and documentation.
View mode hooked into the customize tree.
(view): New custom group.
(view-highlight-face, view-scroll-auto-exit)
(view-try-extend-at-buffer-end)
(view-remove-frame-by-deleting,	view-mode-hook):
Defined by defcustom instead of by defvar.
(view-mode-enter): Install exit-action also when view-mode is
already on. Small rewrite using unless.
(view-mode, view-mode-exit, view-scroll-lines, view-really-at-end)
(view-search): Small rewrite using when or unless.