Correction to the migr. guide wording and removal of not needed option
-Remove the X509 parser sensitivity control for an unknown critical extension from config.h
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+Remove the config option MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION
+--------------------------------------------------------------------------
-It affects users who use the `MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION`
-option which if set, allowed the X509 parser to parse an X509 certificate
-even when it encountered an unknown critical extension.
+This change does not affect users of the default configuration; it only affect
+users who enable this option.
+
+The X.509 standard says that implementations must reject critical extensions that
+they don't recognize, and this is what Mbed TLS does by default. This option
+allowed to continue parsing those certificates but didn't provide a convenient
+way to handle those extensions.
The migration path from that option is to use the
`mbedtls_x509_crt_parse_der_with_ext_cb()` function which is functionally
