Clarify when it's necessary to copy selectors

[courthead]

Previously, it was unclear for people skimming the docs that the advice found in the Sharing Selectors Across Multiple Components section only applies for those trying to solve the specific problem mentioned in the previous section. As a result, I'm sure some people were padding private copies of selectors to different components even in situations where they didn't need to. This makes the documentation more clear.

[codecov-io]

Current coverage is 100.00%

Merging #107 into master will not affect coverage as of 79e8f9b

@@            master    #107   diff @@
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  Files            1       1       
  Stmts           42      42       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit             42      42       
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of 79e8f9b

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[coveralls]

Coverage remained the same at 100.0% when pulling 29891c7 on courthead:patch-1 into 865f7de on reactjs:master.

[ellbee]

Thank you!