[llvm] [LangRef] Clarify the `nsz` semantic for `{u, s}itofp` (PR #201557)

[Jay Foad via llvm-commits]

================
@@ -13192,6 +13195,9 @@ quantity and converts it to the corresponding floating-point value. If the
 value cannot be exactly represented, it is rounded using the default rounding
 mode.
 
+if the '``nsz``' flag is set and the input value is 0, the sign bit of
----------------
jayfoad wrote:

```suggestion
If the '``nsz``' flag is set and the input value is 0, the sign bit of
```

https://github.com/llvm/llvm-project/pull/201557