Document how incorrectly creating schedulers can cause excessive network usage

[Balearica]

When workers are created one at a time for a scheduler, Chrome downloads the code and lang data once for the first worker, and then uses the cached version for the later workers. However, if multiple workers are created in parallel (and the data is not already cached), Chrome appears to download new data for every worker. This could cause significant issues--inflating data usage for mobile users and significantly slowing down Tesseract.js initialization times for users with slow connections. This should be explained in the documentation.