Modify documentation of select() method for Database.

Modify documentation of select() method for Database.

* Add a note in Database.php that docs are in IDatabase.php,
maybe its just me, but I didn't realize that was where the docs
are (even though in retrospect that should have been obvious) and
wasted a lot of time because I didn't realize that.
* Change references to IDatabase::tableName() in doc to
DatabaseBase::tableName(), since tableName is not in the IDatabase
interface.
* Be explicit in docs for select() about which parts are safe for
user input. I think its important to be very explicit about this.

No code changes, just doc changes.

Change-Id: I3a66477afc6a5f54855062482ef04c2966e468f6