Why would that be obvious? There ARE other permutations possible with 15/16 slams, are they not? like 7 Wimbys, 5 USO's, and 3 AO's and 1 FO for instance, so even though numerically no-one else could have done it, Fed may not achieve it either
PS: Besides which, no matter how "obvious" a record may be, it is still normal practice to note it "for the record"; being obvious is not usually a rationale for omission.
Your post makes no sense.
You were asking if Fed winning 5th AO (and thus completing a 3x5) would be a record. It's obvious that it would, because no one but him has won 15 slams, so no one else can possibly have a 3x5.
When you are asking if some achievement is a record, you are really asking: "Has anyone else done this before?" Obviously, no one, in this case.