De-emphasise switch-related keywords in perlfunc.

De-emphasise switch-related keywords in perlfunc.

Move the 3 non-function keywords into the summary cross reference section
at the end of perlfunc. Eliminate the examples which duplicate examples in
perlsyn. Merge the remaining 2 keywords into the "control flow" group.
Note that the switch feature is considered experimental.

With these changes, `perldoc -f default`, `perldoc -f given` and
`perldoc -f when` will still return a helpful result, and as before will
refer the reader to perlsyn for full information.