Document new PyTeal capability: DynamicScratchVar

[michaeldiamant]

algorand/pyteal#198 adds an end-user PyTeal capability called DynamicScratchVar. Since there's a new capability, the issue exists to request demos and docs updates as DevRel sees fit.

Related:

• Update user guide docs to reflect addition of DynamicScratchVar algorand/pyteal#226 - Provides user guide updates.
• https://github.com/algorand/pyteal/blob/master/tests/pass_by_ref_test.py - Provides illustrative usage examples.