Documentation issue: Cleaning up some other types

## Description

Not a bug, just an omission in the documentation. The [cleaning up variables](https://solidity.readthedocs.io/en/v0.6.7/internals/variable_cleanup.html) page doesn't mention the cleanup applied to `bytesN`, `address`, and contract types when needed. Thank you!

Start with the linked cleaning up variables documentation page at solidity.readthedocs.io/en/v0.6.7/internals/variable_cleanup.html. Review the existing cleanup documentation and add the omitted behavior for bytesN, address, and contract types when needed. Done means the page documents these types consistently with the rest of the cleanup section.

Written by the indexing model from the issue text.