Update document of lease and clock.

[xuezhaojun]

When the agent and hub clocks become out of sync, the conditions will be listed as Unknown, and the agent won't update the managed cluster status. This ensures that the system remains accurate and reliable, even in the event of a temporary desynchronization.

Related PR: open-cluster-management-io/ocm#770

[xuezhaojun]

/assign @xuezhaojun