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>

include/mbedtls/threading.h

@@ -46,6 +46,9 @@ extern "C" {
 typedef struct mbedtls_threading_mutex_t
 {
     pthread_mutex_t mutex;
+    /* is_valid is 0 after a failed init or a free, and nonzero after a
+     * successful init. This field is not considered part of the public
+     * API of Mbed TLS and may change without notice. */
     char is_valid;
 } mbedtls_threading_mutex_t;
 #endif