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/mathmodule.c b/Modules/mathmodule.c
index ba84232..8f6a303 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -2759,8 +2759,8 @@
 
 
 PyDoc_STRVAR(module_doc,
-"This module is always available.  It provides access to the\n"
-"mathematical functions defined by the C standard.");
+"This module provides access to the mathematical functions\n"
+"defined by the C standard.");
 
 
 static struct PyModuleDef mathmodule = {