[ Docs]: AsyncIterators

Pre-check

• I know I can edit the docs but prefer to file this issue

Describe the improvement

Missing documentation

Description of the improvement / report

I'm learning the API and stumbled upon the iterator functions provided by $$() (ChainablePromiseArray). I wondered what the difference between map() and mapSeries() were, but did not find any usable information in the Documentation. Neither a search in the WebdriverIO API documentation nor the type definitions (node_modules/webdriverio/build/types.d.ts) held any information. I had to look at the implementation to find the documentation strings: https://github.com/webdriverio/webdriverio/blob/main/packages/wdio-utils/src/pIteration.ts#L31

I suggest that there should be a dedicated Section about the AsyncIterators and that the doc comments are also added to the type definition files.

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