User docs: Improved user contribution mapping

Document improved user contribution mapping feature.

Dev docs will be done with User mapping - Add documentation on how the fea... (#472406 - closed) and/or Dev docs: Document user contribution mapping co... (#449034 - closed).

Document first as experiment.

First version is done in !160763 (merged). We have couple of outstanding comments to take care of !160763 (comment 2021600030). Also:

• add that if limit of placeholder users is reached, all contributions will be assigned to importing user
• In https://docs.gitlab.com/ee/user/project/import/#reassign-contributions-and-memberships it mentions: Contributions and memberships cannot be reassigned to bot users on the destination instance, even if the original contributions were done by bots users. Is it worth saying "bot users/internal users", as it may not be clear what a bot user is, with a link to the docs for Internal Users?