Add C26515/C26816 warning documentation

[Rastaban]

Adding documentation for C++ static analysis warnings C26515/C26816. These were added sometime last year but the documentation somehow got missed :(

[PRMerger18]

Microsoft authors of articles need to make updates through the private repository so the updates can be staged and validated by the current validation rules. After you review staging and fix all validation issues, you can add the sign-off comment to let the PR reviewers know the updates are ready to be merged.It's easy to access the private repo version of your articles in GitHub. Just add -pr to the URL, as follows:
Public repo URL: https://github.com/Microsoft/azure-docs/blob/master/articles/azure-portal-overview.md
Private repo URL: https://github.com/Microsoft/azure-docs-pr/blob/master/articles/azure-portal-overview.md
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