Modernize spam section in Gnus manual slightly

Modernize spam section in Gnus manual slightly

* doc/misc/gnus.texi (The problem of spam): Don't explain what
spam is; there is no need for that in 2022.  Don't explain
limitations of obsolete software TMDA; it's website has stopped
working but seems to have been updated last in 2007.
(Thwarting Email Spam, Anti-Spam Basics)
(Spam Package Configuration Examples): Prefer "spam" to "UCE".
(The problem of spam): Use example.org in example.
(SpamAssassin, Hashcash): Improve wording.