Include information about custom multinomial distribution in GBM

Description

adds support for custom multinomial distribution. Add this example to the User Guide. See changes here for more information:
https://github.com/h2oai/h2o-3/blob/rel-yau/h2o-docs/src/dev/custom_functions.md

Assignee

Angela Bartz

Fix versions

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Reporter

Angela Bartz

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Major
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