doc: explain how type parameters interact with `new` in parametric constructors (#61442)

The constructors documentation used `new` with implicit and explicit
type parameters in examples without clearly stating the underlying rule.
This adds a subsection explaining that `new` inherits the type
parameters from the curly braces on the inner constructor name, when
explicit parameters are needed, and what happens when the constructor
name has fewer parameters than the type requires. The existing
explanation in the "Outer-only constructors" section is also updated to
reference this rule.

The rules described here were determined empirically, as the existing
documentation did not specify them. If any details are inaccurate,
corrections are welcome.

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Co-authored-by: James Wrigley <JamesWrigley@users.noreply.github.com>