Update Compare revisions docs

Problem

Context !183565 (comment 2398932508)

As far as I know, documentation does not exist for the Compare revisions feature as a whole. The limitations of the feature are not surfaced to users in documentation, or on the feature itself.

We have documentation for how to Compare branches, but not for Compare revisions where users can compare:

• branch to a branch
• tag to a branch
• commit to a branch
• commit to a commit

The Learn more about comparing revisions link on the Compare revisions page actually points to the documentation for Compare branches, which can be misleading / not holistic.