java.text.NumberFormat (and subclasses) spec updates

The java.text.NumberFormat class and subclasses could be improved to better convey information. As it stands, it can be seen as a "wall of text" which is off putting to readers.

Certain improvements such as better separating the text into sections with headers, advising to skip the pattern related sections if not needed, and general wording improvements can go a long way in helping readability.

This issue is filed to separate the cleanup changes that were originally going to go into JDK-8327640