Clarify the pre-conditions for the `buf` parameter in `H5Dwrite`

**Reference:**
- [HDF forum thread](https://forum.hdfgroup.org/t/how-to-create-scalar-variable-length-string/10309)
- [`H5Dwrite`](https://docs.hdfgroup.org/hdf5/develop/group___h5_d.html#title36)

Clarify the pre-conditions for the `buf` parameter when writing datasets or attributes with a variable-length string element type and a scalar dataspace. Contrast with fixed-length strings!