Improve docs for ASN.1 bitstrings and their usage
diff --git a/include/mbedtls/x509_csr.h b/include/mbedtls/x509_csr.h
index 0c6ccad..a3c2804 100644
--- a/include/mbedtls/x509_csr.h
+++ b/include/mbedtls/x509_csr.h
@@ -205,6 +205,14 @@
* \param key_usage key usage flags to set
*
* \return 0 if successful, or MBEDTLS_ERR_X509_ALLOC_FAILED
+ *
+ * \note The <code>decipherOnly</code> flag from the Key Usage
+ * extension is represented by bit 8 (i.e.
+ * <code>0x8000</code>), which cannot typically be represented
+ * in an unsigned char. Therefore, the flag
+ * <code>decipherOnly</code> (i.e.
+ * #MBEDTLS_X509_KU_DECIPHER_ONLY) cannot be set using this
+ * function.
*/
int mbedtls_x509write_csr_set_key_usage( mbedtls_x509write_csr *ctx, unsigned char key_usage );