Docs: Make minor improvements in `extend.md`

* Wrap text at 80 characters.
* Rephrase content from the `Apple Touch Icons` section to limit subject
  to only `apple-touch-icon`s (ref: #1486).
* Other improvements.
Docs: Add note about iOS 7.1 `minimal-ui` property

Ref: #1485.
Close: #1485.