[SE-0439] Restore generic where example

[Jager-yoo]

Hi, I noticed the inheritance list associatedtype snippet was duplicated, and the generic where clauses function example from the previous revision had gone missing.

This PR:

• removes the duplicate associatedtype example
• restores the func f<T1, T2>(…) where T1: P1, T2: P2 example
• minor docs cleanup

Thanks for taking a look!
cc @mateusrodriguesxyz, @xwu