Document "text" vs. "bytes" values for Entities

[tseaver]

Docs need to explain clearly how users can ensure that the values they set onto entities get mapped as they expect: "text" values should be mapped to/from 'string_value' entries on the protobufs, and "bytes" values should be mapped to/from 'blob_value' entries.

We are to have to educate folks that the native 'str' is "bytes" on Python2, but "text" on Python3, and mandate that they be explicit if they want to control how such values get stored.

[sheridangray]

It may be helpful to explain to Python2 users that they need to encode their strings with unicode in order to map it correctly. I understood that ""text" values should be mapped to/from 'string_value' entries on the protobufs" but it took me a bit of time to figure out HOW to do that.