Documentation/dev-guide: add gRPC-gateway example of txn target VERSION syntax.

[hexfusion]

Fixes #10113

[codecov-io]

Codecov Report

Merging #10115 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10115      +/-   ##
==========================================
+ Coverage    71.7%   71.72%   +0.02%     
==========================================
  Files         390      390              
  Lines       36269    36269              
==========================================
+ Hits        26006    26014       +8     
+ Misses       8462     8452      -10     
- Partials     1801     1803       +2

Impacted Files	Coverage Δ
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
raft/storage.go	87.96% <0%> (-1.86%)	⬇️
pkg/schedule/schedule.go	81.69% <0%> (-1.41%)	⬇️
mvcc/watchable_store.go	84.21% <0%> (-1.06%)	⬇️
clientv3/leasing/kv.go	89.03% <0%> (-1%)	⬇️
etcdserver/cluster_util.go	58.74% <0%> (-0.9%)	⬇️
clientv3/op.go	73.47% <0%> (-0.87%)	⬇️
etcdserver/server.go	74.02% <0%> (-0.8%)	⬇️
etcdserver/raft.go	80.09% <0%> (-0.72%)	⬇️
etcdserver/v3_server.go	79.02% <0%> (-0.47%)	⬇️
... and 11 more

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[xiang90]

lgtm