Edited contrib.admin check messages for grammar and consistency.
Edited contrib.auth check messages for grammar and consistency.
Cleanup of contrib.contenttypes check messages.
Edited contrib.sites check messages for grammar and consistency.
Edited MySQL-specific check messages for grammar and consistency.
Edited model check messages for grammar and consistency.
Edited model and field checks for grammar and consistency.
Added first cut at reference documentation for the checks framework.
Fixed #22193 -- Made hint a truly optional arugment on check messages.

Thanks to Thomas Güttler for the suggestion.
Added backwards compatibliity checks to reference documentation.
Corrected expected test output to allow for cleanups in signal and compatibility checks.