Document that the value of config.numpy.seterr_* is not mutable.

doc/library/config.txt

@@ -319,6 +319,8 @@ import theano and print the config variable, as in:
     errors, and it can be overriden for specific errors by setting one
     (or more) of the flags below.
 
+    This flag's value cannot be modified during the program execution.
+
 .. attribute:: numpy.seterr_divide
 
     String Value: `'None'`, `'ignore'`, `'warn'`, `'raise'`, `'call'`,
@@ -329,6 +331,8 @@ import theano and print the config variable, as in:
     Sets numpy's behavior for division by zero. 'None' means using the
     default, defined by numpy.seterr_all.
 
+    This flag's value cannot be modified during the program execution.
+
 .. attribute:: numpy.seterr_over
 
     String Value: `'None'`, `'ignore'`, `'warn'`, `'raise'`, `'call'`,
@@ -339,6 +343,8 @@ import theano and print the config variable, as in:
     Sets numpy's behavior for floating-point overflow. 'None' means
     using the default, defined by numpy.seterr_all.
 
+    This flag's value cannot be modified during the program execution.
+
 .. attribute:: numpy.seterr_under
 
     String Value: `'None'`, `'ignore'`, `'warn'`, `'raise'`, `'call'`,
@@ -349,6 +355,8 @@ import theano and print the config variable, as in:
     Sets numpy's behavior for floating-point underflow. 'None' means
     using the default, defined by numpy.seterr_all.
 
+    This flag's value cannot be modified during the program execution.
+
 .. attribute:: numpy.seterr_invalid
 
     String Value: `'None'`, `'ignore'`, `'warn'`, `'raise'`, `'call'`,
@@ -359,3 +367,4 @@ import theano and print the config variable, as in:
     Sets numpy's behavior for invalid floating-point operation. 'None'
     means using the default, defined by numpy.seterr_all.
 
+    This flag's value cannot be modified during the program execution.