Exception timing

It's not clear which exceptions are thrown during the synchronous portion of the call vs. which are applied to the returned task.

For example, ArgumentNullException can be checked for synchronously, so I'd guess that's when it would be thrown; then again, there's nothing stopping the methods from deferring it.

Regarding HttpRequestException, most such exceptions have to be async, but some, such as an immediate check of whether the device is attached to a network, can be done in the sync portion. Does calling code need to be prepared for an HttpRequestException during both the sync and async portions?

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#### Document Details

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* ID: 2d060a3c-fa61-4f7d-771d-9a33c2bb3dd5
* Version Independent ID: 612cd2e0-d2d5-e0f5-6c1d-5ca962a8813d
* Content: [HttpClient.PostAsync Method (System.Net.Http)](https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient.postasync?view=netframework-4.7.2#System_Net_Http_HttpClient_PostAsync_System_Uri_System_Net_Http_HttpContent_System_Threading_CancellationToken_)
* Content Source: [xml/System.Net.Http/HttpClient.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Http/HttpClient.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**

xml/System.Net.Http/HttpClient.xml とリンクされている HttpClient.PostAsync API ドキュメントから始めます。同期呼び出しと返される task について、それぞれどの例外がドキュメント化されているかを追跡し、その後、呼び出し元が各例外型をいつ処理すべきか判断できるように API ドキュメントを更新します。

索引モデルが issue の本文から書いたものです。