Document complexity of System.Linq.Lookup<TKey,TElement>

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**Issue moved from MicrosoftDocs/feedback#3425**
- Please respond to @EricAllen-tr.

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_From @EricAllen-tr on Wednesday, January 27, 2021 10:29:13 PM_

I'm trying to find the approximate algorithmic complexity of Lookup's indexing operation, and it isn't to be found on https://docs.microsoft.com/en-us/dotnet/api/system.linq.lookup-2.item?view=netframework-4.5.1#System_Linq_Lookup_2_Item__0_

I'd like if it were documented, similar to what is done for Dictionary
https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2.item?view=netframework-4.5.1#remarks
> Getting or setting the value of this property approaches an O(1) operation.

I don't want to rely on current testing because current implementations are not contractual. I want to rely on the documented contract.

Apologies if this came in the wrong way. The .NET API Doc doesn't have the handy "click here to improve this page" that the Visual Studio documentation has.