To add, or not to add, a Note to §15.6.10 Extension methods

[Nigel-Ecma]

This is spun off from #1209, where a comment contains:

In the preexisting text I did notice that a reference parameter can be of a generic type constrained to a struct, an input parameter cannot be, and there appears to be no explanation why. The answer can be found in the Readonly references proposal, should something (normative or informative) be added?

This is referring to the bullet list in §15.6.10 Extension methods and the section in extensions and generics in the Readonly references proposal.

Should something normative or informative be added to justify this difference?

[jskeet]

We'll close this, deciding that explaining the reasoning would probably cause more confusion than leaving it unexplained.