simplify some language (pyca#4495)

* simplify some language

* Update reference.rst

* wrap

* remove trailing whitespace

the github web editor is bad and should feel bad

docs/x509/reference.rst

@@ -462,8 +462,13 @@ X.509 Certificate Object
         certificate validation is a complex problem that involves much more
         than just signature checks.
 
-        An example on how to check if a certificate signature is correct with a method
-        on the public key (this does not mean that the cert should be trusted):
+        To validate the signature on a certificate you can do the following.
+        Note: This only verifies that the certificate was signed with the
+        private key associated with the public key provided and does not
+        perform any of the other checks needed for secure certificate
+        validation. Additionally, this example will only work for RSA public
+        keys with ``PKCS1v15`` signatures, and so it can't be used for general
+        purpose signature verification.
 
         .. doctest::