Document that generic functions cannot be commands

* doc/lispref/commands.texi (Defining Commands):
* doc/lispref/functions.texi (Generic Functions): Document
that generic functions cannot be turned into commands.
(Bug#33170)

doc/lispref/commands.texi

@@ -136,6 +136,9 @@ start with a capital, e.g., @code{"use (system-name) instead."}); @code{t}; any
 other symbol, which should be an alternative function to use in Lisp
 code.
 
+Generic functions (@pxref{Generic Functions}) cannot be turned into
+commands by adding the @code{interactive} form to them.
+
 @menu
 * Using Interactive::     General rules for @code{interactive}.
 * Interactive Codes::     The standard letter-codes for reading arguments

doc/lispref/functions.texi

@@ -1345,6 +1345,13 @@ to invoke the other auxiliary or primary methods.
 This allows you to add more methods, distinguished by @var{string},
 for the same specializers and qualifiers.
 @end table
+
+Functions defined using @code{cl-defmethod} cannot be made
+interactive, i.e.@: commands (@pxref{Defining Commands}), by adding
+the @code{interactive} form to them.  If you need a polymorphic
+command, we recommend defining a normal command that calls a
+polymorphic function defined via @code{cl-defgeneric} and
+@code{cl-defmethod}.
 @end defmac
 
 @cindex dispatch of methods for generic function