HBASE-25284 Check-in "Enable memstore replication..." design (#2680)

Update the design document to the state as of not. Adds a paragraph
on the preamble. See discussion on end of HBASE-25284.

Signed-off-by: Duo Zhang <zhangduo@apache.org>
diff --git a/dev-support/design-docs/HBASE-18070-ROOT_hbase:meta_Region_Replicas.pdf b/dev-support/design-docs/HBASE-18070-ROOT_hbase:meta_Region_Replicas.pdf
index 9f992e6..caeafaf 100644
--- a/dev-support/design-docs/HBASE-18070-ROOT_hbase:meta_Region_Replicas.pdf
+++ b/dev-support/design-docs/HBASE-18070-ROOT_hbase:meta_Region_Replicas.pdf
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