Add missing HttpClient doc

[aik-jahoda]

Summary

Add missing HttpClient doc

part of: dotnet/runtime#41446

[carlossanlop]

Thanks for the changes, @aik-jahoda . I left some suggestions for you to consider.

[aik-jahoda]

Reopen to trigger builds

[opbld34]

Docs Build status updates of commit da5b3c5:

✅ Validation status: passed

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

[opbld32]

Docs Build status updates of commit cd2b54a:

✅ Validation status: passed

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

[opbld31]

Docs Build status updates of commit 3077d5d:

✅ Validation status: passed

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