help: fix internals.changegroups Add information about tree manifests, copy edit the text and fix up a few ambiguities. The document also contains a few additional fixes from Siddharth Agarwal <sid0@fb.com>, who used it to build a parser for changegroups in Rust.

share: drop 'relshared' requirement as well

share: fix typo to drop 'shared' requirement on unshare This must be a typo and it seems correct to drop the requirement since the repo is no longer a shared repository.