[7.12] [DOCS] Adds screenshot and short explanation on ROC curve in classification example (#1585)#1587

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szabosteve merged 1 commit intoelastic:7.12from szabosteve:backport/7.12/pr-1585

Feb 23, 2021

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Commits on Feb 23, 2021

[DOCS] Adds screenshot and short explanation on ROC curve in classification example (elastic#1585 )
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