Update Tuples design doc based on implementation and discussion

[WardBrian]

This makes three changes to the tuples design doc:

1. Single element tuples will not be allowed in the initial implementation due to the syntax inconsistency and general uselessness
2. Clears up an ambiguity found during Feature/3161 parse tuples json stan#3165 (comment)
3. Adds a note on constraints and which will be valid inside tuples in the initial implementation