SPARK-3642. Document the nuances of shared variables.

I was going to suggest that this might not be true, but it looks like this is partially-true now that #2524 has been merged.  That PR added a paragraph to the programming guide which clarifies that we guard against duplicate updates only for updates performed inside of actions and _not_ for ones performed in transformations: https://github.com/apache/spark/commit/66cc2431462a5354bb50c196a59da0ffc258c466?diff=unified#diff-3

In light of this, do we still need this paragraph?