Add remark discouraging HttpRequestMessage reuse

[MihaZupan]

Summary

Document that modifying and/or reusing an HttpRequestMesssage instance after it has been sent is discouraged (things may blow up in interesting ways at runtime).

Contributes to dotnet/runtime#66989

[ghost]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Summary

Document that modifying and/or reusing an HttpRequestMesssage instance after it has been sent is discouraged (things may blow up in interesting ways at runtime).

Contributes to dotnet/runtime#66989

Author:	MihaZupan
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	-

[opbld30]

Docs Build status updates of commit 0bda0c1:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http/HttpRequestMessage.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 the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[ManickaP]

Thanks.