Document all 'bug compatibility' features in impl/memory

[riannucci]

@danjacques

There's a lot of weird undocumented stuff in impl/memory which was added after discovering that behavior in production. It would be great to be able to pull each weird thing out into a tiny 'documentation app' which uses the stock go SDK to form a minimal test case which can be uploaded to an appspot instance to demonstrate the bug/limitation. This would also help us remove these weird things when they eventually get fixed (since the goal of impl/memory is to provide an as-accurate-as-possible replication of the production behavior as possible).

Some examples of this include:

• limits on # entities modified in a transaction
• discrepancy between unset namespace and "" namespace in some APIs (like datastore_v3.RunQuery)
• oddball combinations of Query parameters which don't work/behave in surprising ways
• limits on # simultaneously modifiable entities in e.g. PutMulti
• maximum RPC size limits
• etc.

Note that impl/memory currently doesn't implement some of these, as it doesn't actually implement the RPC middleman. If we fix #23, some of these will become more pertinent.