Move the submodule explanation to somewhere?

[JohnTitor]

Correct. The [patch] tables are gone.

BTW, I was noticing that there seems to be a lot of detailed information about git in this "introduction" chapter. Almost nobody needs to know about how to update a submodule. I feel like more of this could be pushed off into the git chapter (or maybe even a subchapter of that). I held back from making larger changes.

Originally posted by @ehuss in https://github.com/rust-lang/rustc-dev-guide/pull/1450#discussion_r957681615