Document using SCIM#Update to change extern_uid for users

SAML and SCIM rely on us having a consistent NameID/extern_uid to identify which user is logging in. When the Identity Provider changes the id it returns this will prevent the user from being able to log in as well as potentially causing other problems such as duplicate users being created by SCIM.

One way to fix this is to use the SCIM API to update extern_uid values using https://docs.gitlab.com/ee/api/scim.html#update-a-single-saml-user