Document further how 'text' vs. 'bytes' values are stored.

Note how the two differ in Python2 vs. Python3.

Fixes #266.
Think-o:  'bytes_value' -> 'blob_value'.

Incorporates feedback from @dhermes.
Merge pull request #331 from tseaver/266-document_text_vs_bytes_values

Fix #266: Document further how 'text' vs. 'bytes' values are stored.