Document `value_mapper`

[philomory]

Description

I'd love for the value_mapper function to be documented.

Acceptance Criteria

The documentation in the docs folder contains documentation for the value_mapper function.

Describe Alternatives

The alternative is to discover this useful function by either reading through various github issues until you stumble upon someone mentioning it, or to end up going to the source code rather than trusting the documentation.

Dependencies

I can't think of any.

Additional context

This function seems very useful! Even if it's not supported, it could at least be documented in the "experimental functions" section.

Estimates

S

For Maintainers Only or Hero Triaging this bug

Suggested Priority (P1,P2,P3,P4,P5):
Suggested T-Shirt size (S, M, L, XL, Unknown):

[workato-integration]

https://new-relic.atlassian.net/browse/NR-304974