R1:3469265e17f9

Improve config option documentation for Imagemagick

Summary: Fixes T7306. Fixes a typo and improves the text.

Test Plan: reading

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7306

Differential Revision: https://secure.phabricator.com/D11797