Lowercase "Really Cool Feature" & minor edits

Here's my changes:
- I thought it was probably wrong that everywhere "Really Cool Feature" was all capitalized.
- I also added a link to a definition of "Mirabile dictu".
- changed a word or two to make it more clear.
- Added a newline before the `Polymorphic functions` heading so that hopefully it will render properly.
Merge pull request #593 from justgage/patch-1

Lowercase "Really Cool Feature" & minor edits