(beginning of emacs-lisp-intro.texi): Add `other shell commands' to

(beginning of emacs-lisp-intro.texi):  Add `other shell commands' to
produce additional output formats; total is now ten.
(A Loop with an Incrementing Counter, and others): Ensure Info menus
will appear in short windows.
(Disentangle beginning-of-buffer): Replace `version 21' with `more
recent versions'.
(Simple Extension): Show how to handle multiple versions by adding
an alternative with a test of `>= 21'