Clarify the introduction of section, "How are Docstring
Examples Recognized", by moving it to above the example
and rewriting for clarity.
Change the example in that section to use more plausible
code, which also demonstrates a multi-line test fixture.
Add three "fine print" points, about the example prefix
strings, blank expected output, and multi-line expected
output.
Clarify the introduction of section, "What's the Execution
Context", by rewriting for clarity, and breaking into three
paragraphs.
Copy-edit the first paragraph in section, "Which Docstrings
Are Examined?".