BUG-648: Document AttributesBuilder expectation

This patch just clarifies the expectation that the attributes map does
not change once it is passed in.

Change-Id: Ifd22f5cad85d97cd6bc8f408ad22ac634ed6df16
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-648: Document AttributesBuilder expectation"