Improve DOCUMENTATION tests

* Use new helper function ASSERT-DOCUMENTATION

* Consistently use WITH-TEST

* Group all DOCUMENTATION tests in one section of the file
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