perf-tuning: add document for predicate push down (#3181)#3721

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ti-srebot wants to merge 1 commit intopingcap:release-3.0from ti-srebot:release-3.0-cedf5df141e2

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Commits on Jun 19, 2020

cherry pick pingcap#3181 to release-3.0

XuHuaiyu
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ti-srebot
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