Update the `GlobalWorkerOptions.workerSrc` JSDoc comment
Update the `GlobalWorkerOptions.workerSrc` JSDoc comment

This particular JSDoc comment is fairly old and it also contains some now unrelated/confusing information.
The only way to *guarantee* that the PDF.js library works as expected is to correctly set the global `workerSrc`[1], hence giving the impression that the option isn't strictly necessary is thus incorrect.

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[1] Since advertising the fallbackWorkerSrc functionality definitely seems like the *wrong* thing to do.

xavier114fch/pdf.js,mozilla/pdf.js,nawawi/pdf.js,macroplant/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,caremerge/pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,caremerge/pdf.js,macroplant/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,xavier114fch/pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js