replaced @file{spam.el} with @code{spam.el}

replaced @file{spam.el} with @code{spam.el}
everywhere for consistency.
(Filtering Spam Using The Spam ELisp Package): admonish again.
(Spam ELisp Package Sequence of Events): this is Gnus, say so.
Say "regular expression" instead of "regex."  Admonish.  Pick
other words to sound better (s/so/thus/).
(Spam ELisp Package Filtering of Incoming Mail): mention
statistical filters.  Remove old TODO.
(Spam ELisp Package Sorting and Score Display in Summary Buffer):
new section on sorting and displaying the spam score
(BBDB Whitelists): mention spam-use-BBDB-exclusive is not a
backend but an alias to spam-use-BBDB
(Extending the Spam ELisp package): rewrite the example using the
new backend functionality.