Interlink/explain differences with lint, fmt and fix goals

[huonw]

This does a few tweaks focused on the docs about the lint, fmt and fix goals:

• explain the difference between them, particularly the "syntactic" (fmt) vs. "semantic" (fix) changes, which I couldn't find expressed anywhere else
• link between them
• expand the Python "linters and formatters" & goal docs to consider fix too
• link to the various subsystems for each Python tool