Add HttpContent missing API

[aik-jahoda]

Summary

Add missing HttpContent documentation

part of: dotnet/runtime#41446

[carlossanlop]

@dotnet/docs do you know why this PR hit all those warnings? Normally the warnings belong to the changes in the PR, not to the whole repo.

[carlossanlop]

Changes look good, @aik-jahoda , thanks for submitting the PR. Let's just wait for @dotnet/docs input on why the build hit all those warnings.

[aik-jahoda]

@dotnet/docs do you know why this PR hit all those warnings? Normally the warnings belong to the changes in the PR, not to the whole repo.

@carlossanlop, it also doesn't publish the preview to the review.docs.microsoft.com

[aik-jahoda]

Reopening to trigger build.

[opbld30]

Docs Build status updates of commit 6b63a8f:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http/HttpContent.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.

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