Adds documentation for `wipe` and `commit` (#5053)

* Adds documentation for `wipe` and `commit`

This adds documentation to `wipe` and `commit` of `Externalities`.
Besides that it removes the default implementation that would just panic
and requires that all implementers of the trait implement the functions.

* Update primitives/externalities/src/lib.rs

Co-Authored-By: joe petrowski <[email protected]>

Co-authored-by: joe petrowski <[email protected]>