I think a good ER diagram is worth a million words ... but I am not personally strong on wanting to document anything. I have found a great tool that helps with database documentation -- schemaspy

So here is my first contribution to the knowledge base of drupal... SchemaSpy Analysis of drupalcvs which is a 4.8/50 CVS install of drupal!!! -- click around -- it is a very cool tool. It would show table and field comments if there were any.

Seems the drupal developers have discussed this for a while. There are starts but since it is node 28,046 and drupal.org is over 80,000 now ... Seems the effort is restarting as of last week Documenting database schema via processing of .install files

Maybe a view like schemaspy makes would help..