[docs] Document git conflict resolution capabilities

[jakub-g]

yarn has a built-in mechanism for resolving git merge conflicts in yarn.lock; just running yarn will resolve the conflicts.

However:

• this is nowhere documented (except some AUTOMERGE error codes), hence not discoverable
• it would be great to know how the algorithm exactly works under the hood.

[proyectoramirez]

Additionally, as of yarn 4, this relies on the project using git. This results in yarn install throwing on hg repositories (which wasn't the case under yarn 3)