Clarify get_ex opcode docs

[barnjamin]

The app_{local|global}_get_ex opcode will return 0 for exists flag if the app or key don't exist.

[jannotti]

Be sure to run make in data/transactions/logic and check in the spec changes that result.

[codecov-commenter]

Codecov Report

Merging #3208 (df8f483) into master (d3bbe62) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3208   +/-   ##
=======================================
  Coverage   47.12%   47.13%           
=======================================
  Files         366      366           
  Lines       58675    58675           
=======================================
+ Hits        27653    27658    +5     
+ Misses      27804    27798    -6     
- Partials     3218     3219    +1     

Impacted Files	Coverage Δ
data/transactions/logic/doc.go	77.41% <ø> (ø)
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	⬇️
network/wsNetwork.go	62.84% <0.00%> (-0.20%)	⬇️
data/transactions/verify/txn.go	44.29% <0.00%> (ø)
network/requestTracker.go	71.12% <0.00%> (+0.86%)	⬆️
network/wsPeer.go	70.83% <0.00%> (+2.22%)	⬆️

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[jannotti]

Thanks.