Document that mongoS no longer pins connections to secondaries

As of server 2.8 and server 2.6.4, mongoS no longer pins secondary connections to allow changes in read preferences between queries.

Special attention, from ticket: "As these changes could not be backported to 2.6, a different fix was implemented specifically for 2.6: a new mongos server parameter, internalDBClientRSReselectNodePercentage was introduced. This can be set to any value from 0 to 100 (defaults to 0) and represents the probability (expressed in percentage) of a replica set connection in mongos to reevaluate replica set node selection from scratch, regardless of the compatibility of the current read preference to the last-used secondary node. Extra care should be taken since reselecting a replica set node will destroy the old connection and create a new connection. This means in extreme cases (for example, 100%), mongos can be creating and destroying connections for every read request."

Text re: pinning at http://docs.mongodb.org/manual/core/read-preference-mechanics/ should be changed.

See linked.