Update Docs on Subroutines and ScratchVariables after the Code for Subroutines and ScratchVariables Stabilizes

[tzaffi]

Problem

Subroutines code is undergoing significant changes and these need to be documented. The main PR's that will be affecting subroutines are:

• Pass-by-Ref / Dynamic Scratch Variables via the loads and stores opcodes #198
• ABI Router implementation and test #170
• Optimize ScratchVars into stack variables #173 (though this may not change the user interface...)
• Support passing ABI types to and from subroutines #179
• Other ABI: Defining an ARC-4 Application #185 issues/PR's

Solution

Write the docs

Dependencies

Yes. See the above.

Urgency

High. Useful, accurate, and up to date, docs are crucial for PyTEAL developers.

[michaeldiamant]

Closing for these reasons:

• Update user guide docs to reflect addition of DynamicScratchVar #226 updates the user guide to reflect DynamicScratchVar.
• For ABI types, we'll update the user guide again once ABI PRs stabilize and before closing out related issues.
• For DevRel updates reflecting DynamicScratchVar, I opened Document new PyTeal capability: DynamicScratchVar  algorandfoundation/docs#770 per DevRel's request.