Changing the migration guide entry wording.
Remove the `MBEDTLS_X509_ALLOW_EXTENSIONS_NON_V3` option
--
-This change does not affect users who are working with current V3 X.509
-certificates.
+This change does not affect users who were using the default configuration, as
+this option was already disabled by default. Also, it does not affect users who
+are working with current V3 X.509 certificates.
-This change makes the pre-V3 X.509 certificates both with or without optional
-extensions obsolete.
+Extensions were added in V3 of the X.509 specification, so pre-V3 certificates
+containing extensions were never compliant. Mbed TLS now rejects them with a
+parsing error in all configurations, as it did previously in the default
+configuration.
If you are working with the pre-V3 certificates you need to switch to the
current ones.
