Updates to design docs to reflect accepted proposals

[josh11b]

Includes proposals:

• Generics details 8: interface default and final members #990
• Pattern matching syntax and semantics #2188
• Checked and template generic terminology #2138
• Template generics #2200
• Types are values of type type #2360
• Consistent class and interface syntax #2760
• Expression phase terminology #2964
• Reduce ambiguity in terminology #3162

Also tries to use more precise language when talking about:

• implementations, to avoid confusing impl declaration and definitions with the impls operator used in where clauses, an issue brought up in Keyword to use in place of is in where...is constraints #2495 and Replace keyword is with impls #2483;
• "binding patterns", like x: i32, and "bindings" like x.

[chandlerc]

Overall looks good. Just a bunch of really minor suggestions, most optional.

LG with or without.