Looks like for a developer no, you'd have to read parser code. For a user, yes:

$ ./pants help-advanced buildgen.go | grep -A5 go-template
--buildgen-go-template=<template> (@fromfile value supported) (default: {{#target.parameters?}}
{{target.type}}(
  {{#target.parameters}}
  {{#deps?}}
  dependencies=[
    {{#deps}}
...