docs: Update description for 'max stat cache'

The size specified here reflects the total size of all caches, not just
the stat cache. It might also make sense to rename this parameter, but
for for now at update the description.

Signed-off-by: Christof Schmitt <cs@samba.org>
23 jobs for christofschmitt-bug13865 in 110 minutes and 42 seconds (queued for 6 seconds)
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