[docs] LibraryEvolution: Address smaller feedback points.

...from JoeG, Dmitri, and David Owens.

- Add an explicit section on default arguments.
- Replace the phrase "permitted but discouraged" with a new and stronger
  term "binary-compatible source-breaking change". This may need more
  work but it is an improvement.
- Mark that we could make converting get-only 'var' to 'let' safe.
- Adding a required convenience init is safe with certain restrictions.
- Add some more related proposals.
- Add a todo list for more substantive changes and ongoing discussion.
- Assorted clarifications and typo fixes.

Author: jrose-apple