I understand this as hdr.Data refers to the underlying pointer in the slice after the assignment to hdr.Data. Notice p in uintptr(unsafe.Pointer(p)):

var s string
hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1
hdr.Data = uintptr(unsafe.Pointer(p))              // case 6 (this case)
hdr.Len = n

In this usage hdr.Data is really an alternate way to refer to the underlying pointer in the slice header, not a uintptr variable itself.

The line should still say string, not slice, because hdr.Data refers to the string header.

However yeah there is also a phantom pointer p to an object (array?) of length n that the string is being redefined to, which makes the doc very hard to read. I would like to see an explicit p added.

I can't come up with a reasonable way to define p and n in this example. The only plausible case is calling a cgo function, but I don't want to have to refer to cgo here.

Change https://golang.org/cl/96983 mentions this issue: unsafe: fix reference to string header