'method' property docs incorrectly say it's optional

Problem/Motivation

The docs say:

> * - method: (optional) What to do if the input file uri does not exist.

AFAICT, the 'method' property is not optional. ProcessPluginBase::transform() will throw an exception if it's not specified.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes