pacific: doc/radosgw: explain multisite dynamic sharding

[zdover23]

Add a note to doc/radosgw/dynamicresharding.rst and a note to doc/radosgw/multisite.rst that explains that dynamic resharding is not supported in releases prior to Reef.

This commit is made in response to a request from Mathias Chapelain.

Co-authored-by: Anthony D'Atri anthony.datri@gmail.com
Signed-off-by: Zac Dover zac.dover@proton.me
(cherry picked from commit d4ed422)

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