expand backend documentation

Add descriptions for PLP::Backend::FastCGI and ::Apache explaining
their configuration and advantages, and recommend/reorder backends
accordingly.

Document a major bug in mod_perl2, and discourage its use for now.
Not investigated, but easily reproducable with:

	<:
	use Cwd qw(cwd);
	say cwd, '<br>';
	sleep 10;  # now request a plp from a different directory
	say cwd, '<br>';