[flang][NFC] Document ambiguous case of DATA in BLOCK

Fortran is not clear about the semantics of

```
  subroutine subr
    integer n = 1
    block
      data n/2/
    end block
  end subroutine
```

which could be interpreted as having two variables, each
named 'n', or as having one variable 'n' with invalid double
initialization.  Precedents from existing compilers are also
in disagreement.

The most common interpretation, however, agrees with a subtle
reading of the standard: BLOCK constructs scope names that have
local specifications, and a DATA statement is a declaration
construct, not a specification construct.  So this example is
*not* acceptable.

Differential Revision: https://reviews.llvm.org/D134391