Add additional documentation for workerPath config

[mogzol]

This PR will...

Add additional documentation around the workerPath config and the ESM distribution.

• Adds a section to MIRATING.md mentioning that workerPath should be included when using the ESM distribution and wanting web workers to be enabled.
• Adds a comment to the code example in API.md about the above
• Adds a note in the workerPath documentation in API.md about the above

Why is this Pull Request needed?

See the comments on #5757

Are there any points in the code the reviewer needs to double check?

No, this is just a small documentation change

Resolves issues:

None. Related to #5757, but does not resolve the actual issue.

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md