raft: clarify conditions in unstable.stableTo

[nvanbenschoten]

No change in behavior, but clarify interaction with unstable snapshot.

Extracted from #14627. cc. @tbg

Signed-off-by: Nathan VanBenschoten nvanbenschoten@gmail.com

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov-commenter]

Codecov Report

Merging #14720 (c18d79d) into main (0bff3ad) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #14720      +/-   ##
==========================================
- Coverage   75.55%   75.48%   -0.08%     
==========================================
  Files         457      457              
  Lines       37323    37324       +1     
==========================================
- Hits        28201    28173      -28     
- Misses       7354     7377      +23     
- Partials     1768     1774       +6     

Flag	Coverage Δ
all	75.48% <100.00%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
raft/log_unstable.go	94.20% <100.00%> (+0.26%)	⬆️
client/pkg/v3/tlsutil/tlsutil.go	83.33% <0.00%> (-8.34%)	⬇️
client/v3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
client/pkg/v3/testutil/leak.go	62.83% <0.00%> (-4.43%)	⬇️
server/etcdserver/txn/util.go	75.47% <0.00%> (-3.78%)	⬇️
server/proxy/grpcproxy/watch.go	93.64% <0.00%> (-2.90%)	⬇️
server/etcdserver/api/v3rpc/maintenance.go	72.78% <0.00%> (-1.37%)	⬇️
client/v3/op.go	74.71% <0.00%> (-1.15%)	⬇️
server/etcdserver/api/v3rpc/watch.go	85.39% <0.00%> (-0.96%)	⬇️
server/storage/mvcc/watchable_store.go	84.42% <0.00%> (-0.73%)	⬇️
... and 7 more

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[ahrtr]

LGTM