Fix first_execution_run_id comments on Cancel and Terminate RPCs

The comments on RequestCancelWorkflowExecution and
TerminateWorkflowExecution claimed that first_execution_run_id
validates against either the most recent or specified (by run id)
execution. In reality, the server unconditionally ignores the run id
when first_execution_run_id is set, always resolving to the most
recent execution for the workflow id.

The UpdateWorkflowExecution comment is left unchanged because its
server implementation does respect the run id when set.

Author: dandavison