Fetch cluster slots from node upon first -MOVED response.

This is a more sane default as it allows users to indicate only a few
servers of the whole cluster composition, while it previously required
a more complex configuration of the client using client options.

This feature can be disabled using the "enableClusterNodes()" method.
Minor phpdocs fixes.

[ci skip]
Fetch updated slots map from node indicated by -MOVED response.

This optimization makes it possible to fetch the slots map directly
from the server indicated by the -MOVED response eliminating the need
to use a random node in the pool, which in turn could require Predis
to open a new and useless connection.
Fix phpdocs.