Matthew Oliver 8df3d1505a Document memcache max size with shard ranges
When you start getting more then 3k shards in a root container the
cached shard range listing can get bigger then the default max size for
memcache (1MB).

So add a mention about it in the configuration guide.

Fixes bug 1890643

Change-Id: If380410c17ed9ebc014b8198af0ea8d502deacc8
2021-01-08 15:24:28 +11:00
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