Doc section on Node Access Rights needs update for D7

The doc section http://api.drupal.org/api/group/node_access/7 needs to be updated for Drupal 7.

This comes from a section that starts with

@defgroup node_access Node access rights

in modules/node/node.module

Known problems with this doc section:
- Doesn't explain the current method for deciding on access rights -- see #481822: Node grants only apply to published nodes, documentation doesn't make this clear (this deals with D6 documentation) and #452538: Enable Node Grants for Unpublished Nodes (this updates how things are done for D7). Actually, you probably want to wait until 452538 has been fixed/committed before updating this doc, but it looks like it's about to happen.
- Mentions db_rewrite_sql(), which no longer exists in D7
- doesn't mention the alter hooks