I noticed the doc/development/testing_guide/best_practices.md still
referenced the `[run mysql]` tags, etc. They no longer work, so I
removed them, then realised I had better clean up the rest of
doc/development !

These attributes are stored in binary in the database, but exposed as
strings. This allows one to query/create data using plain SHA1 hashes as
Strings, while storing them more efficiently as binary.