Improve documentation on default @search argument for String


The documentation seems to says that @search directive on String type must have an argument, as the table of the doc here says. That table has no entry for the none argument for String, while there is one for others types, like Bool and Enum for example.

But, in practice, Dgraph accepts the @search directive with no argument on Strings.
The documentation should reflect it, and expose to what constructed searches it correspond to.