Parallel: clarify javadoc
Parallel: clarify javadoc

Clarify that the op implementing the Parallel interface will spawn
multiple threads -- *not* that the Parallel op is intended to be *used*
simultaneously from multiple threads. (That would be a Threadable op.)

gab1one/imagej-ops,stelfrich/imagej-ops,imagej/imagej-ops,kephale/imagej-ops