Behavior of `branches` filter on existing branches

### Code of Conduct

- [x] I have read and agree to the GitHub Docs project's [Code of Conduct](https://github.com/github/docs/blob/main/.github/CODE_OF_CONDUCT.md)

### What article on docs.github.com is affected?

https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax

### What part(s) of the article would you like to see updated?

These to parts that describe using a `branches` filter:

- [on..](https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#onpull_requestpull_request_targetbranchesbranches-ignore)
- [on.push.](https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#onpushbranchestagsbranches-ignoretags-ignore)

### Additional information

I just lost about 5 hours trying a large variety of variations and reviewing the docs related to `branches` filters on GitHub Actions. Here is what I discovered, and I believe this important fact is nowhere in the docs and should be:

```
When a workflow is put in place that has a `branches` filter, or modified
to add additional branches to an existing `branches` filter, any matching
branches that already existed PRIOR to this modification are NOT eligible
to trigger the workflow.

Only new branches added AFTER the workflow is in place, with matching
branch names, can trigger the workflow.
```