Refine and elaborate text in P3273

[bernhardmgruber]

Hi! @andralex asked me to read through P3273 (Introspection of Closure Types) and provide feedback. This PR contains a few wording improvements.

Furthermore, here are some additional suggestions:

• "all implementations already implement by-reference lambda captures by means of reference or pointer members". I am very curious about which implementations use pointers. This could also be interesting to other readers, so I suggest to explicitly add a table with the implementations you had a look at.
• "we anticipate no or small impact on compiler implementations.". Are there any implications for users? A closure type using pointer data members for capture by reference could be (trivially) copyable, so changing the pointers to references could break this. I also wonder whether the ABI could be effected if the data member types would be changed and old and new object code would be mixed. Furthermore, I vaguely remember that making a type trivially copyable could change whether it is passed to functions through registers or on the stack. If anybody knows this, I think it would be good to add a sentence on those kinds of potential breakages. Also for these questions it would be interesting which implementations would need to change their data types and which don't.