Add module docstring

Author: JukkaL

mypyc/lib-rt/vecs/librt_vecs.c

@@ -903,7 +903,7 @@ static PyModuleDef_Slot librt_vecs_module_slots[] = {
 static PyModuleDef vecsmodule = {
     .m_base = PyModuleDef_HEAD_INIT,
     .m_name = "vecs",
-    .m_doc = "vecs doc",
+    .m_doc = "This module implements the vec type and related functionality.",
     .m_size = 0,
     .m_methods = VecsMethods,
     .m_slots = librt_vecs_module_slots,