man: tc.8: mention available qdiscs

man: tc.8: mention available qdiscs

Some qdiscs still lack a manpage, so listing them here is the only way
for a user to get to know them. For the others, this serves as an
overview of what is there.

Content was taken over from the dedicated manpage if available and
suitable, so there is definitely room for improvement at least by
adjusting it more to the context in which it is now. In case there
wasn't appropriate wording available, I tried to identify key aspects of
the given qdisc.

Signed-off-by: Phil Sutter <phil@nwl.cc>