[SPARK-9819][STREAMING][DOCUMENTATION] Clarify doc for invReduceFunc in incremental versions of reduceByWindow

- that reduceFunc and invReduceFunc should be associative
- that the intermediate result in iterated applications of inverseReduceFunc
  is its first argument

Author: François Garillot <francois@garillot.net>

Closes #8103 from huitseeker/issue/invReduceFuncDoc.