git: Document resolving references doesn't verify object existence
When passing a reference to
git rev-parse, it'll resolve that
reference to an object ID. That ID mustn't necessarily be a commit, in
fact not even a tag, but could in fact be anything. The most interesting
edge case is also that "anything" also includes "nothing". As a result,
our use of
ResolveRefish doesn't actually verify that any given object
ID actually exists, but really only works at the reference-level.
While this behaviour actually makes sense, it certainly can be unexpected. So let's document this behaviour and show ways to actually resolve it to a certain object type, which will then also check for existence.