add UI help text to clarify flush for memacached (was: Flushing memcached bucket does not set Item Count to 0)

When a memcached bucket is flushed through the UI, the Item Count does not become 0.

Steps to reproduce:

• Create a memcached bucket with Flush enabled.
• Put data in the memcached bucket so that Item Count in the UI is > 0.
• Using the UI, flush the memcached bucket.
• Note that Item Count is > 0.
• Note that if Gets are performed on the data, the Item Count will be reduced.

My understanding is that when a flush is run on a memcached bucket, data is not actually removed, but all items are set to expired. Since the data is not actually removed, it may be technically correct for the Item Count to remain non-zero, however it is confusing to report this in the UI.

Suggestion is that either Item Count be set to 0 following a successful flush of memcached buckets since they are 'effectively' removed, or that a note of help / explanation be linked to or otherwise provided following the flush of a memcached bucket.