Update composition.md to clarify split method

[srubin20170623]

TODO:

• [ ] Make sure all of new logic is covered by tests and passes linting
• [ ] Update CHANGELOG.md with your change

[apollo-cla]

@srubin20170623: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[codecov-io]

Codecov Report

Merging #1245 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1245   +/-   ##
======================================
  Coverage    95.6%   95.6%           
======================================
  Files          22      22           
  Lines        1116    1116           
  Branches      174     162   -12     
======================================
  Hits         1067    1067           
  Misses         44      44           
  Partials        5       5

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