Document the extra_data argument to Backend.parse_answer

This has been there since forever, but didn't make it into the custom backends
guide for some reason.
Hiera 1.3: Document lookup functions in interpolation tokens

This commit also makes references to interpolation throughout the manual more
agnostic about what's being interpolated, as it may not necessarily be variables
anymore.
Remove "coming soon" space for a puppet backend example

It's not coming soon, sorry.
Hiera 1.3: you can now interpolate into hash keys
Hiera 1.3: Release notes (for RC2)