pg_isready docs doesn't specify what exactly it is checking

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/14/app-pg-isready.html
Description:

`pg_isready` docs don't specify how its different from a port probe - what
does on top of checking that a port is open? Does it send some handshake
message that a server responds to when initialization is finished?

People here are telling that `pg_isready` is not reliable and that
`pg_isready` may return when database is not actually ready
https://github.com/docker-library/postgres/issues/880#issuecomment-1004251102
Without further info about `pg_isready` it is not clear if it should be
filled as a bug. Or if it should be filled as a bug only when an
initialization script is involved.