[doc] clarify units for linkcheck_rate_limit_timeout (#12225)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

This normalizes the description of the configuration value so that its default value's unit is the same as the internal one (i.e., seconds).

doc/usage/configuration.rst

@@ -2939,7 +2939,8 @@ Options for the linkcheck builder
 
    Otherwise, ``linkcheck`` waits for a minute before to retry and keeps
    doubling the wait time between attempts until it succeeds or exceeds the
-   ``linkcheck_rate_limit_timeout``. By default, the timeout is 5 minutes.
+   ``linkcheck_rate_limit_timeout``. By default, the timeout is 300 seconds
+   and custom timeouts should be given in seconds.
 
    .. _Retry-After: https://datatracker.ietf.org/doc/html/rfc7231#section-7.1.3