Document why size < 0 can happen

Author: marcelm

Modules/_io/bytesio.c

@@ -530,6 +530,7 @@ _io_BytesIO_peek_impl(bytesio *self, Py_ssize_t size)
     n = self->string_size - self->pos;
     if (size < 1 || size > n) {
         size = n;
+        /* size can be negative after truncate() */
         if (size < 0) {
             size = 0;
         }