update docs about leave parameter

[littlezz]

It seems that leave == True means keep, but old docs say it means remove

[codecov-io]

Current coverage is 100.00%

Merging #142 into master will not affect coverage as of 0892622

@@            master    #142   diff @@
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  Files            7       7       
  Stmts          351     351       
  Branches        66      66       
  Methods          0       0       
======================================
  Hit            351     351       
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of 0892622

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