Document the need to call psa_crypto_init() with USE_PSA_CRYPTO

When MBEDTLS_USE_PSA_CRYPTO is enabled, the application must call
psa_crypto_init() before directly or indirectly calling cipher or PK code
that will use PSA under the hood. Document this explicitly for some
functions.

To avoid clutter, this commit only documents the need to call
psa_crypto_init() in common, non-obvious cases: parsing a public key
directly or via X.509, or setting up an SSL context. Functions that are
normally only called after such a function (for example, using an already
constructed PK object), or where the need for PSA is obvious because they
take a key ID as argument, do not need more explicit documentaion.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/ssl.h

@@ -1869,6 +1869,10 @@ void mbedtls_ssl_init(mbedtls_ssl_context *ssl);
  *                 Calling mbedtls_ssl_setup again is not supported, even
  *                 if no session is active.
  *
+ * \note           If #MBEDTLS_USE_PSA_CRYPTO is enabled, the PSA crypto
+ *                 subsystem must have been initialized by calling
+ *                 psa_crypto_init() before calling this function.
+ *
  * \param ssl      SSL context
  * \param conf     SSL configuration to use
  *