Minor fixes.

Minor fixes.
Replace all tabs with eight spaces each so printed text looks correct.
Remove extraneous comma in a printed node name produced by `ref'.
(insert-buffer): Add a missing beginning parenthesis.
(beginning-of-buffer): Add `beginning of' to note about accessible portion.
(narrow Exercise): Write closing parenthesis at end of correct paragraph.
(zap-to-char): Remove extraneous `a' from first sentence.
(Complete zap-to-char): Remove two extraneous sentences.
(zap-to-char body): Move sentences on documentation two nodes earlier.
(Lisp macro): Add definition of `unless' macro.
(last-command & this-command): Remove comment that `we have not yet
seen' the @code{eq} function.
(kill-append function): Reformat `kill-append' function definition so
it prints well.
(kill-new function): Indent the sentence beginning `notice'.  Replace
`the same as' with `similar to'.  Repair typo.  Remove obsolete
references to `yank' and `yank-pop.  End section with a note that `we
will digress into C.'