DOC Rearrange selector sections

[redapple]

Same motivations as for scrapy/parsel#79

[codecov-io]

Codecov Report

Merging #2705 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2705   +/-   ##
=======================================
  Coverage   84.66%   84.66%           
=======================================
  Files         162      162           
  Lines        9122     9122           
  Branches     1353     1353           
=======================================
  Hits         7723     7723           
  Misses       1141     1141           
  Partials      258      258

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