Since I updated the validation engine[1], it became possible to create
results with siblings. This commit changes the "DateTimeDiff", allowing
it to create a result with a sibling when possible. That will improve
the clarity of the error messages.
While at it, I noticed that we were not translating the type of
interval, so I fixed that and improved the documentation.
[1]: 238f2d506a
They only know what message a rule produces when they try running it.
Including the messages in the documentation will make it easier for
people to know what to expect.
There are a couple of undocumented placeholders, but I can live with
that for now.
Besides the interface's name, everything already calls this type "Rule",
not "Validatable." This commit puts a stone on it and renames the
interface for better naming.
The method has been renamed some time ago, but I haven't updated the
documentation to cause less confusion. Now that I want to start updating
the documentation, I switched the default branch on GitHub to the latest
stable version, so I assume there will be less confusion in the
documentation, and I can start updating the docs for the next version.
