doc: Tweak documentation of error IDs (bug #66015)

* error.cc (Ferror): Use @code rather than @var around function name.  User
"ERROR-NAME" rather than "WARNING-NAME".  Use template for error message in
example that starts with "fcn_name" to hint that this should be the name
of the function.

libinterp/corefcn/error.cc

@@ -1193,17 +1193,17 @@ by @samp{error: }.
 
 The optional @var{id} argument allows programmers to tag an error
 with a specific identifier so that users can later retrieve it (using
-@code{lasterr} or @var{lasterror}) and know the origin of the error.
-The identifier must contain at least one colon character (@qcode{":"})
+@code{lasterr} or @code{lasterror}) and know the origin of the error.
+The identifier must contain at least one colon character (@qcode{':'})
 and must not contain any whitespace characters.  It should be a string of
-the form @qcode{"NAMESPACE:WARNING-NAME"}.  Octave's own errors use the
+the form @qcode{"NAMESPACE:ERROR-NAME"}.  Octave's own errors use the
 @qcode{"Octave"} namespace (@pxref{XREFerror_ids,,@code{error_ids}}).
 For example:
 
 @example
 @group
 error ("MyNameSpace:wrong-type-argument",
-         "foo: argument should be numeric");
+       "fcn_name: argument should be numeric");
 @end group
 @end example