Answer
There isn't an equivalent reference documentation source for PL/SQL API functions — the REST/OData APIs and the underlying PL/SQL APIs are related but not a 1:1 mapping, and API Explorer only documents the REST side. For PL/SQL functions without REST equivalents, reading the actual package source (connecting to the IFS database with a PL/SQL client like PL/SQL Developer and inspecting the relevant package body) is the practical way to understand exactly what a given function does, its parameters, and its return value.