clientv3/lease.go: TTL, document expired Lease.

[hexfusion]

Document the output of TTL when a Lease is expired. fixes (#9033)

[codecov-io]

Codecov Report

Merging #9034 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9034      +/-   ##
==========================================
- Coverage   76.12%   75.98%   -0.15%     
==========================================
  Files         359      359              
  Lines       29838    29838              
==========================================
- Hits        22715    22673      -42     
- Misses       5548     5591      +43     
+ Partials     1575     1574       -1

Impacted Files	Coverage Δ
clientv3/lease.go	92.77% <ø> (ø)	⬆️
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
auth/range_perm_cache.go	55.88% <0%> (-11.77%)	⬇️
pkg/adt/interval_tree.go	83.48% <0%> (-6.91%)	⬇️
auth/simple_token.go	87.03% <0%> (-6.49%)	⬇️
etcdserver/api/v3rpc/watch.go	88.58% <0%> (-4.57%)	⬇️
mvcc/watchable_store.go	85.97% <0%> (-4.43%)	⬇️
clientv3/leasing/cache.go	87.77% <0%> (-3.89%)	⬇️
clientv3/leasing/kv.go	89.93% <0%> (-1.68%)	⬇️
rafthttp/peer.go	90.22% <0%> (-1.51%)	⬇️
... and 12 more

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