Document mutex usage for RSA

The mutex is now initialized iff ver != 0.

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

ChangeLog.d/rsa-mutex.txt

@@ -6,3 +6,8 @@ Bugfix
    * Fix a resource leak in a bad-arguments case of mbedtls_rsa_gen_key()
      when MBEDTLS_THREADING_C is enabled on platforms where initializing
      a mutex allocates resources.
+
+Default behavior changes
+   * In mbedtls_rsa_context objects, the ver field was formerly documented
+     as always 0. It is now reserved for internal purposes and may take
+     different values.

include/mbedtls/rsa.h

@@ -124,7 +124,10 @@ extern "C" {
  */
 typedef struct mbedtls_rsa_context
 {
-    int ver;                    /*!<  Always 0.*/
+    int ver;                    /*!<  Reserved for internal purposes.
+                                 *    Do not set this field in application
+                                 *    code. Its meaning might change without
+                                 *    notice. */
     size_t len;                 /*!<  The size of \p N in Bytes. */
 
     mbedtls_mpi N;              /*!<  The public modulus. */
@@ -154,6 +157,7 @@ typedef struct mbedtls_rsa_context
                                      mask generating function used in the
                                      EME-OAEP and EMSA-PSS encodings. */
 #if defined(MBEDTLS_THREADING_C)
+    /* Invariant: the mutex is initialized iff ver != 0. */
     mbedtls_threading_mutex_t mutex;    /*!<  Thread-safety mutex. */
 #endif
 }