[doc] perlclass is not linked to in obvious places

[Altreus]

Where https://perldoc.perl.org/functions/class

https://perldoc.perl.org/perlobj

Description functions/class mentions the concept of a class but doesn't link to it, but if you search for "class" on ppo you just get back to where you came from, so it's hard to find what it's talking about.

perlobj should link to perlclass just out of practicality, although justifiably we might not want to do that while it's still experimental.