Add missing option (auxname) in pac_model documentation.

(cherry picked from commit 2e8ced89)

doc/manual/source/the-model-file.rst

@@ -14755,6 +14755,11 @@ simply add the exogenous variables to the PAC equation (without the weight
     used if the target shows up in level in the auxiliary model,
     equation (A.74) in *Brayton et alii (2000)*.
+    .. option:: auxname = STRING
+
+    Name the auxiliary variable, created by the preprocessor, that
+    will define the expectation term in the PAC equation.
+
 .. operator:: pac_expectation (NAME_OF_PAC_MODEL);