Add remaining missing .NET 9 docs for System.Collections

[buyaa-n]

Summary

Add remaining missing .NET 9 docs for System.Collections area

Fixes dotnet/runtime#105976

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-collections

[learn-build-service-prod]

Learn Build status updates of commit 13eb226:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Collections.Concurrent/ConcurrentDictionary2+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Frozen/FrozenDictionary2+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Frozen/FrozenSet1+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Generic/Dictionary`2.xml	✅Succeeded	View
xml/System.Collections.Generic/Dictionary2+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Generic/HashSet`1.xml	✅Succeeded	View
xml/System.Collections.Generic/HashSet1+AlternateLookup1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 167cbc9:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Collections.Concurrent/ConcurrentDictionary2+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Frozen/FrozenDictionary2+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Frozen/FrozenSet1+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Generic/Dictionary`2.xml	✅Succeeded	View
xml/System.Collections.Generic/Dictionary2+AlternateLookup1.xml	✅Succeeded	View
xml/System.Collections.Generic/HashSet`1.xml	✅Succeeded	View
xml/System.Collections.Generic/HashSet1+AlternateLookup1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel