Updated description of Data.ConnectionPool module

Its mostly expadned description from .cabal and README file. In addition
to those mentioned files it tells what difference it makes to use this
package instead of streaming-commons or conduit-extra.
Documentation for sections "[Constructing] Connection Pool"

In section Connection Pool we describe what we understand under
connection pool family and what are the limitations of our
implementation.

Constructing Connection Pool now tells how to construct different
connection pools and how to define common connection pool parameters
(i.e. ResourcePoolParams).