WiredTiger's metadata file should be documented and read-only to applications.

If an application wants to review the objects in the WiredTiger database, it has to open the metadata file and read through it ("file:WiredTiger.wt").

We don't document that, and we should, it's reasonable for applications to want to do that.

Second, we should enforce a read-only semantic on application handles open on the metadata file, it would be bad if an application modified the file.