Add to documentation what is the migration bot user for

The following discussion from !30738 (merged) should be addressed:

• @abuerer started a discussion: (+1 comment)

  @fjsanpedro Is there any documentation on this user out there? We are starting to see tickets surrounding it and would like to have documentation to link.

  Also, will customers see issues if they remove this user from their instance? If so, what issues would you anticipate?

  Customer ticket: https://gitlab.zendesk.com/agent/tickets/169672 (internal use)