need a discussion on how counters are represented

Currently, the docs are a bit light on the concepts behind counters.  The basic idea is that SDKs are JSON oriented and SDKs have high-level features behind the counter() operations to make dealing with JSON backed counters simple.  This turns into memcached binary protocol incr/decr at the low level.

It would be useful if these concepts were described in the docs.

Concepts:

• how to initialize counters
• how counters work with get() and upsert() like other JSON
• how not to use items you're going to use the counter() functions against (e.g., don't sent them to a JSON object)
• how to override counters as discussed in JCBC-1156  (canonically, overwrite it with a JSON number)

And anything else Brett Lawson can think of (please comment!)