Document exit code -13 and exit code -3

[novusnota]

The exit code -13 is thrown for every message sent to a contract with null code and null data (deployed on the mainnet already).

https://tonviewer.com/transaction/dbd76c11fadb4a69a2f17ddaf8285be3f2836090d374e75e5cbeb876467cbdff

The exit code -3 isn't an exit code for the whole transaction, but rather it's a value that RUNVM instruction pushes on the stack in case the actual stack depth after its execution was less than the specified r integer. See: https://docs.ton.org/v3/documentation/tvm/specification/runvm

Thus, the latter should have a note on top saying that it doesn't apply in usual cases (or somesuch)

[Shvandre]

every message sent to a contract with null code and null data

IIUC null code is enough