Clarifies point in experiment metrics docs.
Clarifies point in experiment metrics docs.

Explains that experiments should use different property IDs for different deployments.
lmorchard/idea-town,lmorchard/testpilot,fzzzy/testpilot,mozilla/idea-town-server,lmorchard/testpilot,lmorchard/idea-town-server,mozilla/idea-town,lmorchard/idea-town,lmorchard/testpilot,meandavejustice/testpilot,meandavejustice/testpilot,chuckharmston/testpilot,lmorchard/idea-town-server,lmorchard/idea-town,chuckharmston/testpilot,lmorchard/testpilot,fzzzy/testpilot,mozilla/idea-town,flodolo/testpilot,meandavejustice/testpilot,lmorchard/idea-town-server,fzzzy/testpilot,flodolo/testpilot,lmorchard/idea-town-server,chuckharmston/testpilot,mozilla/idea-town,mozilla/idea-town-server,fzzzy/testpilot,mozilla/idea-town-server,flodolo/testpilot,meandavejustice/testpilot,flodolo/testpilot,lmorchard/idea-town,chuckharmston/testpilot,mozilla/idea-town-server,mozilla/idea-town