[CSL-1161] Add comments to 'Arbitrary' instance
[CSL-1161] Add comments to 'Arbitrary' instance

This commit adds some comments to explain the absence of a 'shrink'
implementation in an 'Arbitrary' instance, and explain another
non-generic one.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl