[Documentation] Controller logic for defaults Experiment API parameters

[andreyvelich]

/kind feature

We should document that parallelTrialCount, maxTrialCount and maxFailedTrialCount parameters can be omitted in experiment yaml and explain how Katib works in that case.

Default value for parallelTrialCount is 3, defined here.

If maxTrialCount is omitted, experiment is running with parallelTrialCount until ObjectiveGoal is reached or MaxFailedTrialCount is reached, if proper fields are specified.

In addition, we should explain other default Experiment parameters.

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[andreyvelich]

/area release
/priority p1