Add help for the formula structure input for non-linear regression

[HeuristicLab-Trac-Bot]

Issue migrated from trac ticket # 3036

milestone: HeuristicLab 3.3.17 | component: Algorithms.DataAnalysis | priority: medium

2019-10-14 11:52:00: @gkronber created the issue

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The information should include a list of all allowed functions, the grammar as well as usage examples.