docs: Update spec wrt. polymorphism (#791)

* Move "Operation Extensionsibility" to after Type System (and up one
level), rename to "Extension System"
* Add Polymorphism section inside Type System: any `Function` type may
be polymorphic
* Clarify working of operations, including adding Appendix 3 with full
details binary `compute_signature`
* Add `OpaqueOp` as a dataflow node operation
 
fixes #790