Copyedits and updates for Emacs manual's first few chapters.

Copyedits and updates for Emacs manual's first few chapters.

* doc/emacs/basic.texi (Inserting Text): Add xref to Completion.  Add
ucs-insert example, and document prefix argument.
(Moving Point): Fix introduction; C-f/C-b are no longer equivalent
to left/right.  Tweak left-char and right-char descriptions.
M-left and M-right are now bound to left-word/right-word.
(Erasing): Document delete-forward-char.

* doc/emacs/commands.texi (User Input): Define "input event" more clearly.
(Keys): Add xref to Echo Area.
(Commands): Clarify relation between commands and functions.

* doc/emacs/entering.texi (Entering Emacs): Define "startup screen".
Document window-splitting behavior with command-line inputs.
(Exiting): Remove obsolete paragraph about shells without suspend
functionality.

* doc/emacs/frames.texi (Non-Window Terminals): Index just "text-only
terminal", which is used throughout the manual now.

* doc/emacs/screen.texi (Screen, Menu Bar): Copyedits.
(Point): Remove duplicate paragraph on cursors, also in Screen.
(Mode Line): Trailing dashes no longer shown on X displays.