Separated proposed changes to extension semantics from PR #386 and us…

…e TypeError, per @jyasskin (#389)

Major polishing of definition and exposition of extensions by selfissued - yay, thx!  includes:

* Separated proposed changes to extension semantics from PR #386 and use TypeError, per @jyasskin

* Added client data descriptions to all extensions. Accepted suggestions by @jyasskin and @vijaybh.

* Addressed comments by @jyasskin in issue #270

* Gave distinct names to extension inputs and outputs to make descriptions more precise.

* Corrected indexing errors

* Addressed additional comments by Jeff Hodges and Jeffrey Yasskin