putio: handle rate limit errors

For rate limit errors, "x-ratelimit-reset" header is now respected.

docs/content/putio.md

@@ -127,3 +127,12 @@ Properties:
 - Default:     Slash,BackSlash,Del,Ctl,InvalidUtf8,Dot
 
 {{< rem autogenerated options stop >}}
+
+## Limitations
+
+put.io has rate limiting. When you hit a limit, rclone automatically
+retries after waiting the amount of time requested by the server.
+
+If you want to avoid ever hitting these limits, you may use the
+`--tpslimit` flag with a low number. Note that the imposed limits
+may be different for different operations, and may change over time.