bpo-36908: 'This module is always available' isn't helpful. (#13297)

Makes the documentation of math and cmath module
more helpful for the beginners.
diff --git a/Modules/cmathmodule.c b/Modules/cmathmodule.c
index 8319767..b421f04 100644
--- a/Modules/cmathmodule.c
+++ b/Modules/cmathmodule.c
@@ -1232,8 +1232,8 @@
 }
 
 PyDoc_STRVAR(module_doc,
-"This module is always available. It provides access to mathematical\n"
-"functions for complex numbers.");
+"This module provides access to mathematical functions for complex\n"
+"numbers.");
 
 static PyMethodDef cmath_methods[] = {
     CMATH_ACOS_METHODDEF