I keep going in circles on this! Where is the information specific to 'definitionExpression' and 'layerDefinition? what exactly needs to be supplied when updating the JSON for a web map item? In the API reference I can find nothing about these for the update() method [and the examples given for update() appear to be identical to what's provided for save()]. All of the examples on these forums utilize both of these keys:values such as this thread here and here.
Is there a section of the API reference I've missed?