HBASE-18948: Added a note in the Tag implementation details in security.adoc
HBASE-18948: Added a note in the Tag implementation details in security.adoc

Coprocessors that run server-side on RegionServers can perform get and set operations on cell Tags.
Tags are striped out at the RPC layer before the read response is sent back, so clients do not see these tags.

Signed-off-by: Josh Elser <08e628f9d7ca1b20fe79ef383bd8c50d12051375@apache.org>

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