Clarify wording in spec for character groups (#618)

* Remove line tabulation, form feed from whitespace
* Rename newline to line feed or line ending
* Reword spec to be more explicit about whitespace
* Rename `Punctuation` to `Unicode punctuation`
* Reword description of line breaks
* Link unicode punctuation
* Clarify link whitespace by describing "link information"
* Clarify link destination and title

Co-authored-by: Johel Ernesto Guerrero Peña <[email protected]>