Add reference to mbedtls_ecp_set_max_ops() to docs

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index d371e1a..80bf5c9 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -4165,6 +4165,12 @@
  *                              or equals as regards to ops executed in a
  *                              single call is implementation defined.
  *
+ * \note                        For keys in local storage when no accelerator
+ *                              driver applies, please see also the
+ *                              documentation for \c mbedtls_ecp_set_max_ops(),
+ *                              which is the internal implementation in these
+ *                              cases.
+ *
  * \warning                     With implementations that interpret this number
  *                              as a hard limit, setting this number too small
  *                              may result in an infinite loop, whereby each