tagging: document why fixing this horror is hard.

The tagging support module has an "API" that includes other objects directly
interacting with the way it stores data, including at the edges where we
serialize to the wire format for transport.

So, we can't change it. Even if it sucks - it allocates lots of objects, it
uses very inefficient deduplication mechanisms - we are kind of stuck,
right now. Fun times.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>