Docs: add notes on group SSO user management

What does this MR do?

Add some clarification and notes on user accounts and user management for group SAML SSO.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index == N/A
• Crosslink the document from other subject-related docs == N/A
• Feature moving tiers? Make sure the change is also reflected in features.yml == N/A
• Correctly apply the product badges and tiers == N/A
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails == N/A

Review checklist

• Your team's review (required) / assigning to @lyle
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards

/cc @gl-docsteam