clarify warning about small target values with MAPE (fixes #3608) #3615
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See #3608 for more details.
If you use LightGBM's built-in MAPE metric, target values where
abs(label) < 1
are rounded to 1.0 when calculating that metric. LightGBM checks for such values and warns about them.This PR proposes a change to that warning message that I think makes it a bit clearer.