Instead of allowed types, the docstring demonstrates tested types.
Ie, as used within the package being documented, a is always Int64. There is a guarantee this has been tested.
Other types could still work, but similar ones probably result in degraded performance. Eg, something like String15 vs String or Vector vs MVector. Or it may error, the docstring makes no claim about that.
Is it a bad idea for docstring to only indicate how the function is used within the package itself?