Document that buildVariables is the child environment, not parameters

I found the description of RunWrapper.getBuildVariables a bit incomplete/confusing:

> buildVariables
>
> for a non-Pipeline downstream build, offers access to a map of defined build variables; for a Pipeline downstream build, any variables set globally on env

so I suggest changing it, appending:

> Child Pipeline jobs can use this to report additional information to the parent job by setting variables in <code>env</code>. Note that build parameters are not shown in buildVariables.