py/modio: io.BufferedWriter: Describe flushing policy.

py/modio.c

@@ -69,6 +69,12 @@ STATIC mp_uint_t bufwriter_write(mp_obj_t self_in, const void *buf, mp_uint_t si
             return org_size;
         }
 
+        // Buffer flushing policy here is to flush entire buffer all the time.
+        // This allows e.g. to have a block device as backing storage and write
+        // entire block to it. memcpy below is not ideal and could be optimized
+        // in some cases. But the way it is now it at least ensures that buffer
+        // is word-aligned, to guard against obscure cases when it matters, e.g.
+        // https://github.com/micropython/micropython/issues/1863
         memcpy(self->buf + self->len, buf, rem);
         buf = (byte*)buf + rem;
         size -= rem;