DOCUMENTATION works on instances of FUNCALLABLE-STANDARD-CLASS
Previously, methods on DOCUMENTATION and (SETF DOCUMENTATION) were
specialized on STANDARD-CLASS, excluding FUNCALLABLE-STANDARD-CLASS.
While this was in line with the method signatures listed in the CLHS
entry for DOCUMENTATION, that entry also states (for DOC-TYPE equal to
TYPE):
If x is a symbol, returns the documentation string of the class whose
name is the symbol x [...]
indicating that (documentation CLASS-NAME 'type) should work for all
kinds of classes.
Specializing the four methods on CLASS instead STANDARD-CLASS is
1) sufficient to enable operation on FUNCALLABLE-STANDARD-CLASS
2) safe since the accessed %DOCUMENTATION slot is located in CLASS