Review comments on PDF19

Fix link to PDF Technology Notes.

I suggest deleting "Visual browsers can display characters and scripts
correctly. Media players can show captions correctly." from the second
paragraph of the description. These aren't relevant to PDF (especially the
visual browser part, since PDF controls the display of characters and
scripts in other ways).

In the test procedure, specify that the screen readers supports switching
between the two languages:
"Read the PDF document with a screen reader that supports the language of
the phrase and the language of the surrounding text,"