Updated doc strings

Updated the doc strings for SgEntity.beforeCommit(), SgEntity.afterCommit() and
the global before/after commit functions.  The arg sgBatchData was said to be a
dict when in fact it is a list of dictionaries.  Also fixed gibberish text
"to pass data user".
Fixed connections caching Entities that are dirty and shouldn't be

Because Entitiy objects still persist after a connection clears its cache
Entities that are dirty had to ability to re-add themselves back into the cache.
When an Entity instructs a connection to cache itself the connection checks if
the cache is empty and also if another object has been created to represent the
Entity.  If either case is True then the Entity will not be allowed to cache and
SgConnection._cacheEntity() will simply return.