Issue apache#2909 Document stream fields as wrapped Java objects in the JavaScript transform

[sramazzina]

Stream fields are added to the Rhino scope through Context.toObject(), so they reach the script as wrapped Java objects instead of JavaScript primitives. This makes a Boolean field always truthy and makes == / != between two fields compare object identity rather than values.

Add a "Stream fields are Java objects" section to the JavaScript transform documentation describing the behaviour and the .valueOf() / parseInt() workarounds, including the null-safety check.

Fixes #2909

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