Add descriptions to the schema table definitions

The schema.module now supports displaying documentation included in the schema data structure as specified in http://drupal.org/node/146939. We should add this information for all core tables. There are 64 such tables so if a handful of developers document the tables they are most familiar with it will all get done very quickly.

So that we do not have many people trying to edit the same patch file, just add the information in a followup to this issue like this:

node: Base table for all nodes.
node.nid: Node identifier. All tables that reference a specific node join to this field.

Please include table references in curly-brackets, like this:

node_access.nid: The {node}.nid that this rule applies to, or 0 if it applies to all nodes.

I will combine all responses into a single patch. The patch will change the .schema files but only by adding documentation so I assume this will be acceptable even in code freeze.