Update and edit pass through generic design docs (#965)

Includes a variety of changes:

Int -> i32
this -> me
expand terminology doc and add links to it
fix text to reflect inline external impl introduced in Support external impl in class and adapter scopes. #905
no longer have plans for runtime type parameters
style updates like removing parentheticals and "we"
observe is a "declaration" not a "statement", since it can appear outside function bodies
many individual updates, clean-ups, and fixes

Co-authored-by: Richard Smith <richard@metafoo.co.uk>