Update reference.rst:load_der_x509_certificate to note ValueError raised (#12364)

* Update reference.rst:load_der_x509_certificate to note error raised

It raises the error and therefore should be noted in the documentation.

```
>>> x509.load_der_x509_certificate(b64.b64decode("foobar".encode()))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: error parsing asn1 value: ParseError { kind: InvalidLength }
```

* Update docs/x509/reference.rst

Thanks!

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

---------

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

Author: ohemorange

docs/x509/reference.rst

@@ -223,6 +223,9 @@ Loading Certificates
 
     :returns: An instance of :class:`~cryptography.x509.Certificate`.
 
+    :raises ValueError: If a certificate cannot be parsed from the provided
+        data.
+
 Loading Certificate Revocation Lists
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~