Issue #12049: improve RAND_bytes() and RAND_pseudo_bytes() documentation

Add also a security warning in the module random pointing to ssl.RAND_bytes().
diff --git a/Doc/library/random.rst b/Doc/library/random.rst
index f0c4add..52419a1 100644
--- a/Doc/library/random.rst
+++ b/Doc/library/random.rst
@@ -43,6 +43,12 @@
 uses the system function :func:`os.urandom` to generate random numbers
 from sources provided by the operating system.
 
+.. warning::
+
+   The generators of the :mod:`random` module should not be used for security
+   purposes, they are not cryptographic. Use :func:`ssl.RAND_bytes` if you
+   require a cryptographically secure pseudorandom number generator.
+
 
 Bookkeeping functions: