About a recently-added message

A recent commit added the following message:

> "wal_level" must be >= logical.

The use of the term "logical" here is a bit confusing, as it's unclear
whether it's meant to be a natural language word or a token. (I
believe it to be a token.)

On the contrary, we already have the following message:

> wal_level must be set to "replica" or "logical" at server start.

This has the conflicting policy about quotation of variable names and
enum values.

I suggest making the quoting policy consistent.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center