BuildDiff: document what the inputs mean

Dependencies

• [2] NYIIFA32 BuildDiff: split out from JobsetEval GET impl

Change contents

• edit in src/lib/Hydra/Helper/BuildDiff.pm at line 25
  [2.405]

  [2.405]
  + # $builds is the list of current builds, and $builds2 is the list of previous (compared-to) builds