[docs] Clarify limitations section of SourceBasedCodeCoverage.rst
[docs] Clarify limitations section of SourceBasedCodeCoverage.rst

Mention that the code coverage tool becomes less precise whenever
unpredictable changes in control flow occur.

Thanks to Sean Silva for pointing this out!

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@271902 91177308-0d34-0410-b5e6-96231b3b80d8

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