message: Add source positions to syntax docs
diff: @@ -120,3 +120,14 @@ In all three cases, these keys may be missing and the semantics are identical to
  An Effect Operation is like a Value Operation but it does not produce a value.
  It also has the optional `args`, `funcs`, and `labels` fields.
  +
  +Source Positions
  +----------------
  +
  +All syntax objects can optionally have a `pos` field to reflect a source position:
  +
  + { ..., "pos": {"row": <int>, "col": <int>} }
  +
  +The `pos` object has two keys: `row` (the line number) and `col` (the column number within the line).
  +Front-end compilers that generate Bril code may add this information to help with debugging, but tools can't require it to exist (or to follow any specific rules).
  +The [text format parser](tools/text.md), for example, can optionally add source positions.
  