ARROW-5301: [Python] update parquet docs on multithreading
ARROW-5301: [Python] update parquet docs on multithreading

https://issues.apache.org/jira/browse/ARROW-5301

@pitrou I reused some formulation from the csv reader

Author: Joris Van den Bossche <jorisvandenbossche@gmail.com>

Closes #4326 from jorisvandenbossche/ARROW-5301-parquet-docs and squashes the following commits:

7cceb42ac <Joris Van den Bossche> ARROW-5301:  update parquet docs on multithreading

wesm/arrow,cpcloud/arrow,icexelloss/arrow,majetideepak/arrow,wesm/arrow,apache/arrow,wesm/arrow,renesugar/arrow,majetideepak/arrow,icexelloss/arrow,cpcloud/arrow,icexelloss/arrow,kou/arrow,majetideepak/arrow,icexelloss/arrow,majetideepak/arrow,renesugar/arrow,apache/arrow,majetideepak/arrow,xhochy/arrow,kou/arrow,cpcloud/arrow,xhochy/arrow,xhochy/arrow,icexelloss/arrow,kou/arrow,majetideepak/arrow,wesm/arrow,laurentgo/arrow,xhochy/arrow,apache/arrow,majetideepak/arrow,kou/arrow,renesugar/arrow,xhochy/arrow,apache/arrow,laurentgo/arrow,icexelloss/arrow,cpcloud/arrow,apache/arrow,renesugar/arrow,cpcloud/arrow,renesugar/arrow,icexelloss/arrow,renesugar/arrow,wesm/arrow,laurentgo/arrow,apache/arrow,xhochy/arrow,laurentgo/arrow,apache/arrow,icexelloss/arrow,kou/arrow,majetideepak/arrow,kou/arrow,laurentgo/arrow,xhochy/arrow,renesugar/arrow,laurentgo/arrow,renesugar/arrow,icexelloss/arrow,cpcloud/arrow,apache/arrow,majetideepak/arrow,xhochy/arrow,laurentgo/arrow,apache/arrow,wesm/arrow,cpcloud/arrow,laurentgo/arrow,xhochy/arrow,icexelloss/arrow,cpcloud/arrow,laurentgo/arrow,wesm/arrow,majetideepak/arrow,wesm/arrow,renesugar/arrow,renesugar/arrow,apache/arrow,renesugar/arrow,cpcloud/arrow,laurentgo/arrow,xhochy/arrow,apache/arrow,laurentgo/arrow,xhochy/arrow,cpcloud/arrow,wesm/arrow,renesugar/arrow,kou/arrow,xhochy/arrow,wesm/arrow,apache/arrow,kou/arrow,majetideepak/arrow,wesm/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,xhochy/arrow,laurentgo/arrow,xhochy/arrow,icexelloss/arrow,majetideepak/arrow,renesugar/arrow,icexelloss/arrow,kou/arrow,kou/arrow,cpcloud/arrow,wesm/arrow,kou/arrow,majetideepak/arrow,renesugar/arrow,kou/arrow,cpcloud/arrow,renesugar/arrow,wesm/arrow,xhochy/arrow,apache/arrow,laurentgo/arrow,cpcloud/arrow,cpcloud/arrow,wesm/arrow,kou/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,majetideepak/arrow,majetideepak/arrow