[SPARK-12638][API DOC] Parameter explanation not very accurate for rdd function "aggregate"

Currently, RDD function aggregate's parameter doesn't explain well, especially parameter "zeroValue".
It's helpful to let junior scala user know that "zeroValue" attend both "seqOp" and "combOp" phase.

Author: Tommy YU <tummyyu@163.com>

Closes #10587 from Wenpei/rdd_aggregate_doc.