GEODE-4624 Add docs for 2 new gateway sender statistics (#1856)
GEODE-4624 Add docs for 2 new gateway sender statistics (#1856)

* GEODE-4624 Add docs for 2 new gateway sender statistics

* GEODE-4624 Revise new stat wording, per review

PurelyApplied/geode,davebarnes97/geode,davinash/geode,davebarnes97/geode,smgoller/geode,deepakddixit/incubator-geode,masaki-yamakawa/geode,jdeppe-pivotal/geode,masaki-yamakawa/geode,masaki-yamakawa/geode,davinash/geode,jdeppe-pivotal/geode,masaki-yamakawa/geode,PurelyApplied/geode,PurelyApplied/geode,deepakddixit/incubator-geode,deepakddixit/incubator-geode,deepakddixit/incubator-geode,pdxrunner/geode,pdxrunner/geode,pdxrunner/geode,pdxrunner/geode,deepakddixit/incubator-geode,davinash/geode,masaki-yamakawa/geode,davinash/geode,davebarnes97/geode,PurelyApplied/geode,smgoller/geode,PurelyApplied/geode,jdeppe-pivotal/geode,davinash/geode,davebarnes97/geode,deepakddixit/incubator-geode,jdeppe-pivotal/geode,davebarnes97/geode,pdxrunner/geode,davinash/geode,PurelyApplied/geode,davebarnes97/geode,deepakddixit/incubator-geode,davebarnes97/geode,davinash/geode,masaki-yamakawa/geode,jdeppe-pivotal/geode,PurelyApplied/geode,smgoller/geode,pdxrunner/geode,pdxrunner/geode,masaki-yamakawa/geode,smgoller/geode,smgoller/geode,jdeppe-pivotal/geode,smgoller/geode,jdeppe-pivotal/geode,smgoller/geode