Full proofreading pass on user guide.

- Fixed many small typos
- Fixed many small grammar issues, requiring small rephrasing
- Reworded many phrases to avoid problem with "literal"s being truncated
at EOL
- Added blank columns to some tables to avoid overwrites
- Added newlines to code samples to avoid truncation
- Edited all task dependency graphs to use UML "depends on" notation,
with arrow from task to depended task, along with a mention of this in
the introduction
- Added info to "README.md" to describe how to edit graph images
- Fixed template link for API references
- Ensured text uses "left double quote" and "right double quote" where
appropriate
- Made a handful of small actual content changes to emphasize points not
fully covered
- Changed some "p" elements to "para"
- Changed some "thread" elements to "thead"