Clarify the documentation of db_rewrite_sql() [no functional change]

I find the documentation of db_rewrite_sql() a bit vague and propose the following changes:

-- add the {node_type} table to the list
-- clarify $primary_field
-- clarify $args as "not currently used"

Same changes also for _db_rewrite_sql(), except the third (don't know whether it applies).

If I understand this correctly, then single-node accesses should use node_access() for checking permissions. If this is correct, then adding a pointer to node_access() (and possibly vice versa) would be helpful, but I don't know how to add this properly. @see doesn't seem to work correctly.