MINOR: Clarify doc on consumption of topics
MINOR: Clarify doc on consumption of topics

In doc it stays:

_"Our topic is divided into a set of totally ordered partitions, each of which is consumed by one consumer at any given time."_

And consumer is described as:

_"We'll call **processes** that subscribe to topics and process the feed of published messages **consumers**."_

Which might lead to a wrong conclusion - that each partition can be read by one process at any given time.

I think this statements misses information about **consumer groups**, so i propose:

_"Our topic is divided into a set of totally ordered partitions, each of which is consumed by exactly one consumer (from each subscribed consumer groups) at any given time"_

This contribution is my original work and I license the work to the project under the project's open source license.

Author: pilo <jakub.pilimon@4finance.com>

Reviewers: Jiangjie Qin <becket.qin@gmail.com>, Jason Gustafson <jason@confluent.io>

Closes #1900 from pilloPl/minor/doc-fix

sebadiaz/kafka,KevinLiLu/kafka,rhauch/kafka,mihbor/kafka,sslavic/kafka,noslowerdna/kafka,Chasego/kafka,gf53520/kafka,Ishiihara/kafka,sebadiaz/kafka,sebadiaz/kafka,eribeiro/kafka,TiVo/kafka,themarkypantz/kafka,lindong28/kafka,sebadiaz/kafka,ollie314/kafka,rhauch/kafka,rhauch/kafka,richhaase/kafka,airbnb/kafka,ijuma/kafka,themarkypantz/kafka,ErikKringen/kafka,sslavic/kafka,Esquive/kafka,ollie314/kafka,Esquive/kafka,apache/kafka,Ishiihara/kafka,airbnb/kafka,gf53520/kafka,richhaase/kafka,Ishiihara/kafka,richhaase/kafka,guozhangwang/kafka,sslavic/kafka,apache/kafka,KevinLiLu/kafka,lindong28/kafka,noslowerdna/kafka,lindong28/kafka,richhaase/kafka,Chasego/kafka,eribeiro/kafka,guozhangwang/kafka,geeag/kafka,MyPureCloud/kafka,ijuma/kafka,Esquive/kafka,ErikKringen/kafka,MyPureCloud/kafka,mihbor/kafka,geeag/kafka,TiVo/kafka,apache/kafka,guozhangwang/kafka,ijuma/kafka,MyPureCloud/kafka,ijuma/kafka,themarkypantz/kafka,mihbor/kafka,TiVo/kafka,KevinLiLu/kafka,airbnb/kafka,Chasego/kafka,noslowerdna/kafka,themarkypantz/kafka,Chasego/kafka,lindong28/kafka,ErikKringen/kafka,noslowerdna/kafka,eribeiro/kafka,Ishiihara/kafka,KevinLiLu/kafka,ollie314/kafka,apache/kafka,Esquive/kafka,geeag/kafka,mihbor/kafka,gf53520/kafka,ErikKringen/kafka,eribeiro/kafka,guozhangwang/kafka,gf53520/kafka,MyPureCloud/kafka,airbnb/kafka,ollie314/kafka,rhauch/kafka,TiVo/kafka,geeag/kafka,sslavic/kafka