index.hdr.in: add code markup
index.hdr.in: (hopefully) non-controversial edits
index.hdr.in: slightly more controversial changes

 - Clarify wording in functions section
 - Use the term aliases rather than wrappers
 - Clarification of concepts and better? examples in variable expansion
 - Likewise in environment variables
 - Using the phrase builtin commands rather than builtins
 - Tidy up keybindings a bit
 - Another example in the history section
 - Remove TODO section in favour of mailing list, GitHub and IRC link