Add SSO Team IDs to Terraform Enterprise documentation

[JarrettSpiker]

Updates the Terraform Enterprise Team Membership Mapping
documentation to explain then new SSO Team IDs feature

This new functionality expands an organizations to automatically add users to teams through SAML assertions, by allowing the configured SAML attribute to match against a chosen "SSO Team ID" instead of just the team's name in TFE. This is helpful when the configured SAML attribute cannot be a human readable team name due to restrictions in the organization's SSO

See #2210 for the corresponding change to Terraform Cloud documentation

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[mpminardi]

Will we need to update the API docs touched in the previous PR to say something along the lines of SSO team ID is returned if your Terraform Cloud organization is business tier or if you are using Terraform Enterprise?

[laurapacilio]

Some wording nits - thank you!

[laurapacilio]

previous PR

Yes - I think probably we should do that so users aren't confused when reviewing the data that is returned by the API.

[radditude]

Would it make sense to switch this PR to be against #2210? Then we could build off the changes to the API docs in that PR, and once #2210 is merged, Github will switch the base branch back to master.