Improve key export API and documentation

- "master secret" is the usual name
- move key block arg closer to the related lengths
- document lengths

Also fix some trailing whitespace while at it
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 5834279..9c96a05 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1181,7 +1181,7 @@
 /**
  * \def MBEDTLS_SSL_EXPORT_KEYS
  *
- * Enable support for exporting key block and master key.
+ * Enable support for exporting key block and master secret.
  * This is required for certain users of TLS, e.g. EAP-TLS.
  *
  * Comment this macro to disable support for key export