* In light of the composite rings feature being added [1],
downgrade the warnings about EC Duplication [2] being
experimental.
* Add links from Global EC docs to composite rings and
per-policy proxy config features.
* Add discussion of using EC duplication with composite
rings.
* Update Known Issues.
[1] Related-Change: I0d8928b55020592f8e75321d1f7678688301d797
[2] Related-Change: Idd155401982a2c48110c30b480966a863f6bd305
Change-Id: Id97a4899255945a6eaeacfef12fd29a2580588df
9665252352