Consolidate connection customization documentation

1. move all discussion of how to customize connection
to the engines.rst section.   have the section in pooling.rst
point to engines.

2. ensure all of the common use cases for connection modification
are listed out in order of level of modification.   the use
case of modifying an existing connection is separate from that of
modifying how connect is called; ensure that poolevents.connect
is referred to just as prominently as dialectevents.do_connect.

3. completely replace any discussion of create_engine.creator, as this
hook does not offer anything beyond what do_connect() does.
"creator" is more about using a Pool object directly without an
Engine, which is no longer a documented use case.

Change-Id: Ibe366d2a6e63eb420a6136fdc71ce0fb545edf8f

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