Fixes to documentation (language & contributor modules)

Just a few more documentation fixes – this time for the language and contributor module documentation.

- Use backticks consistently for inline code
- Capitalize section headers and proper nouns
- Fix mismatched inline code markup in `list*` usage (this was causing the remaining portion of the document to be parsed incorrectly by syntax highlighters)
- Other miscellaneous grammatical fixes
\`\`hy_\`\`