doc: Fix incorrect documentation in template data model

Per the TemplateData spec:
* Most message values can be null, though they're never undefined.
* 'deprecated' is actually {boolean|string}.

For us:
* Make #isParameterDeprecated always return a boolean
* Add #getParameterDeprecationDescription to get the explanation
  of why a parameter is deprecated
* getParameterDescription doesn't need to pass 'null' for fallback
  since that's the default fallback.
  And it may return null or string.
* #getParameterSets returns an array of Set objects, not an array
  of strings (see spec for more information).

Change-Id: Iee788bc8fa6f0545085a0f154b77b8176f770dd2
Merge "doc: Fix incorrect documentation in template data model"