Improved docstring for mg_read()

mongoose.h

@@ -239,6 +239,10 @@ void mg_send_file(struct mg_connection *conn, const char *path);
 
 
 // Read data from the remote end, return number of bytes read.
+// Return:
+//   0     connection has been closed by peer. No more data could be read.
+//   < 0   read error. No more data could be read from the connection.
+//   > 0   number of bytes read into the buffer.
 int mg_read(struct mg_connection *, void *buf, size_t len);