Add additional asciify documentation

[emiham]

Description

Fixes #929.

This was proposed but never dealt with, so after going through some confusion around this myself I figured it's about time.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Awesome; this will be helpful, I think! Thanks for adding it.