Change the augmentation feature spec according to language issue 3967 (#3980)

Change the augmentation feature spec according to language issue 3967. Several changes were made:

Clean up the text to use ASCII double quotes rather than "smart" double quotes.
Add commentary at several locations to clarify 'augmentation declaration', 'introductory declaration', introduce the notion of the context of an augmentation declaration, adjust the definition of above/below, before/after, clarify that abstract and external variables are just getters/setters and not variables, clarify type inheritance, clarify the rules about `augmented`, clarify various error messages, make terminology more consistent (avoid references to 'original' and 'base' declarations, preferring 'augmenting' and 'augmented' as well as 'introductory' and 'augmentation' declarations). Finally, an example using 'import augment' and 'augment library' was updated to use 'part' and 'part of'.

Author: eernstg