- Change PREFIX to PREFIXES, which contains a sequence of prefix strings.
  This is useful since we want to look for both Py and PY.
- Wrap a long line.
- Collect struct tags as well as typedef names.  Since we generally only
  use one of the other, that improves coverage.
- Make the script executable on Unix.

This could use a better approach to determine if a symbol is documented,
and could easily avoid keeping the massive string in memory.  That would
take time to actually write more code, though, so we'll bail on that
for now.