mutable elements in HashSet

It appears the documentation for `HashSet` should include the same caveat as `Dictionary`: As long as an object is used as a key in the `Dictionary`, it must not change in any way that affects its hash value. Every key in a `Dictionary` must be unique according to the dictionary's equality comparer.

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#### Document Details

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* ID: 6b66ab1a-35aa-1bba-388c-30d698c1f50c
* Version Independent ID: d7dde1b9-fd16-dd30-6622-a15836579a9c
* Content: [HashSet<T> Class (System.Collections.Generic)](https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.hashset-1?view=netframework-4.7.2#remarks)
* Content Source: [xml/System.Collections.Generic/HashSet`1.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Collections.Generic/HashSet`1.xml)
* Product: **dotnet-api**
* GitHub Login: @mairaw
* Microsoft Alias: **mairaw**