Add missing documentation for SerializeToStreamAsync in the networking area

[aik-jahoda]

part of: dotnet/runtime#41446

[opbld32]

Docs Build status updates of commit ce2d2bd:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http/ByteArrayContent.xml	✅Succeeded	View

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[opbld33]

Docs Build status updates of commit ed91056:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http/ByteArrayContent.xml	✅Succeeded	View
xml/System.Net.Http/FormUrlEncodedContent.xml	✅Succeeded	View
xml/System.Net.Http/HttpContent.xml	✅Succeeded	View
xml/System.Net.Http/MultipartContent.xml	✅Succeeded	View
xml/System.Net.Http/MultipartFormDataContent.xml	✅Succeeded	View
xml/System.Net.Http/StreamContent.xml	✅Succeeded	View
xml/System.Net.Http/StringContent.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching in the Docs contributor and Admin Guide
• See the frequently asked questions
• Post your question in the Docs support channel

[BillWagner]

Thanks for writing this @aik-jahoda

It LGTM, and I'll now.