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: using a PK object that was
not constructed using PSA, X.509 processing, or setting up an SSL context.
Functions that are normally only called after such a function (for example,
using a cipher or PK context constructed from a PSA key), 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>
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 235e00c..466611f 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -283,6 +283,10 @@
  * \brief          Parse a single DER formatted certificate and add it
  *                 to the end of the provided chained list.
  *
+ * \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 chain    The pointer to the start of the CRT chain to attach to.
  *                 When parsing the first CRT in a chain, this should point
  *                 to an instance of ::mbedtls_x509_crt initialized through
@@ -344,6 +348,10 @@
  * \brief            Parse a single DER formatted certificate and add it
  *                   to the end of the provided chained list.
  *
+ * \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 chain      The pointer to the start of the CRT chain to attach to.
  *                   When parsing the first CRT in a chain, this should point
  *                   to an instance of ::mbedtls_x509_crt initialized through
@@ -394,6 +402,10 @@
  *                 temporary ownership of the CRT buffer until the CRT
  *                 is destroyed.
  *
+ * \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 chain    The pointer to the start of the CRT chain to attach to.
  *                 When parsing the first CRT in a chain, this should point
  *                 to an instance of ::mbedtls_x509_crt initialized through
@@ -434,6 +446,10 @@
  *                 long as the certificates are enclosed in the PEM specific
  *                 '-----{BEGIN/END} CERTIFICATE-----' delimiters.
  *
+ * \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 chain    The chain to which to add the parsed certificates.
  * \param buf      The buffer holding the certificate data in PEM or DER format.
  *                 For certificates in PEM encoding, this may be a concatenation
@@ -458,6 +474,10 @@
  *                 of failed certificates it encountered. If none complete
  *                 correctly, the first error is returned.
  *
+ * \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 chain    points to the start of the chain
  * \param path     filename to read the certificates from
  *