doc: clarify dd sparse detection is by *output* block

doc: clarify dd sparse detection is by *output* block

The wording of the dd --help text suggests that output will be skipped
for sparse *input* blocks (i.e. that NUL-checking is done on input
blocks) while the code actually checks/skips all-NUL *output* blocks.[1]

* src/dd.c (usage): Update the --help text to clarify the above.
* tests/dd/sparse.sh: Ensure sparseness is controlled with obs.

[1]: https://superuser.com/a/1136358