Update generics overview (#3061)

This reflects changes from a number of approved proposals:

- #2138 : "generic" -> "checked generic", "template" -> "template
generic"
- #2360 : "type", "facet type", "facet". Note: I am not using the term
"generic type" from #2360 since that meaning conflicts with the
generally accepted meaning of "generic type" of a type with a
compile-time parameter.
- #2760 / #2770 : internal/external impl -> extending impl
- #2964 : "symbolic constant" and "template constant"

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