Mark some internal `PDFDocumentProxy`-properties as "private"
Mark some internal `PDFDocumentProxy`-properties as "private"

These two properties were *never* intended to be anything but "private", hence it really cannot hurt to actually indicate that they're *not* part of any official API.

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