HttpRequestHeaders.TryAddWithoutValidation needs description on what it does.

### Type of issue

Typo

### Description

Current description does not cover details. Base on method name I am pretty sure it adds a header if not defined, but documentation does explain what happens if the header is defined. Also I am not sure "Without Validation" means, is there any validation done on headers? I can see for certain types.

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### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.net.http.headers.httpheaders.tryaddwithoutvalidation?view=net-9.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Net.Http.Headers/HttpHeaders.xml

### Document Version Independent Id

baebd354-6fce-5e31-e17f-d101ce91e22d

### Platform Id

89b0fc54-158b-75e7-156d-7a591811ae22

### Article author

@karelz