Copyedit generics.md and traits.md

Squashed at reviewer's request:

Add heading at the end of the introductory material
Spice up introductory paragraphs a bit
Use quotes instead of <code> for phrase
Remove "other" in "other restrictions" (it's not obvious that any other
restrictions have been mentioned)
"Default methods" is a second-level heading, but is not a subsection of
"Where clause"
Reword "Default methods" introduction: it's not the "last feature" on
this page

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