Section 3.1.6 (Named Function References) contains the following text:

<quote>
Certain functions in the [XQuery and XPath Functions and Operators 3.1] specification are defined to be polymorphic. These are denoted as accepting parameters of "numeric" type, or returning "numeric" type. Here "numeric" is a pseudonym for the four primitive numeric types xs:decimal, xs:integer, xs:float, and xs:double. For the purposes of named function references, these functions are regarded as taking arguments and producing results of type xs:anyAtomicType, with a type error raised at runtime if the argument value provided is not of the correct numeric type.

Note:

The above way of modeling polymorphic functions is semantically backwards compatible with XPath 2.0. An implementation that supports static typing can choose to model the types of these functions more accurately if desired.
</quote>

This is obsolete and can be deleted. Since xs:numeric has been defined as a standard union type, there is nothing special about these functions that needs to be said here.