Article should note limit of 100 labels applied to issues and pull requests

[janiceilene]

What feature or product is affected?

https://help.github.com/en/github/managing-your-work-on-github/applying-labels-to-issues-and-pull-requests

What is the new or expected behavior?

A user can apply up to a maximum of 100 labels to issues and pull requests.

How is the old or inaccurate behavior currently documented?

Currently there is no mention of this in the above help article. I would recommend that we add a note to the article noting this limit.

Who does this affect?

Any user who uses labels with issues and pull requests.

What is the impact to users?

If a user applies more than 100 labels to an issue or pull request this can cause problems in the GitHub UI, such as problems closing the issue or pull request.

Content to update

In https://help.github.com/en/github/managing-your-work-on-github/applying-labels-to-issues-and-pull-requests:

• Add content about limits to top of page (before procedural)
• Rather than having two tips/notes on this page, we should just turn the current tip into content.