Indicate on document selection screen that PDFs must have selectable text

## Background
In a discussion of edu/LMS support tickets that have come in since September, PDF issues (specifically, instructors trying to upload and annotate PDFs that don't have selectable text) were among the most common.

We have a KB article that explains [how to ensure PDFs have selectable text](https://web.hypothes.is/help/how-to-ocr-optimize-pdfs/) and we do mention this while onboarding partners. However, we still get a high volume of support requests around this. In addition to a "cultural" fix, we'd like to employ a product-based solution.

## User story
As an instructor using the Hypothesis LMS app with my students, I'm not familiar with the concept of OCR for PDFs, and I don't necessarily know to check whether my PDFs have selectable text before creating a Hypothesis-enabled assignment.

## Brainstorming
One way to approach this could be to add something to our assignment creation screen:

Something to the effect of: "PDFs must have selectable text..." with a link to [our KB article](https://web.hypothes.is/help/how-to-ocr-optimize-pdfs/).

This would cut down on the number of instructors creating assignments with non-annotate-able PDFs.