Add documentation to the main sync code (#2879)

* Added some documentation and question on the main sync code

* Remove strange formatting change

Co-Authored-By: André Silva <[email protected]>

* Improve comment per suggestion

* Remove comment about re-using PeerInfo, no real reason and no real consensus if it's better or worse.

* Fix redundancy in comment

Co-Authored-By: André Silva <[email protected]>

* Explaining more things and fixing review comments.

* Better explanation of MAJOR_SYNC_BLOCKS
* Explains the ancestor search strategy a bit
* Corrects explanation of what a stale block is
* Adds a bit of context for `request_ancestry` function

* Revert code change to not confuse the PR