Since I updated the validation engine[1], it became possible to create
results with siblings. This commit changes the "NullOr", allowing it to
create a result with a sibling when possible. That will improve the
clarity of the error message.
I also updated the documentation, since it was still called "Nullable"
[1]: 238f2d506a
This commit will rename the "Nullable" rule to "NullOr" while soft
deprecating the old name. It should work the same as the previous one
but with a different name. It will also prefix the result ID, allowing
more message customization.
While working on it, I realized that the prefix "nullOr" had a typo,
and it was using "nullOf" instead. I fixed that, too.
Signed-off-by: Henrique Moody <henriquemoody@gmail.com>
