Explain the usage of is_valid in pthread mutexes

Document the usage inside the library, and relate it with how it's
additionally used in the test code.

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

library/threading.c

@@ -67,6 +67,12 @@ static void threading_mutex_init_pthread( mbedtls_threading_mutex_t *mutex )
     if( mutex == NULL )
         return;
 
+    /* A nonzero value of is_valid indicates a successfully initialized
+     * mutex. This is a workaround for not being able to return an error
+     * code for this function. The lock/unlock functions return an error
+     * if is_valid is nonzero. The Mbed TLS unit test code uses this field
+     * to distinguish more states of the mutex; see
+     * tests/src/threading_helpers for details. */
     mutex->is_valid = pthread_mutex_init( &mutex->mutex, NULL ) == 0;
 }