etcdctl: Update README to clarify newline syntax in TXN

[jutley]

Add documentation to clarify that when writing TXN commands, multi-line values should be written using "\n" and not a literal newline (as in other commands).

Fixes #10169

[codecov-io]

Codecov Report

Merging #10513 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10513      +/-   ##
==========================================
+ Coverage    71.4%   71.55%   +0.15%     
==========================================
  Files         393      393              
  Lines       36503    36503              
==========================================
+ Hits        26064    26119      +55     
+ Misses       8604     8553      -51     
+ Partials     1835     1831       -4

Impacted Files	Coverage Δ
etcdctl/ctlv3/command/role_command.go	66.93% <0%> (-4.04%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
proxy/httpproxy/director.go	80% <0%> (-2.86%)	⬇️
etcdserver/api/rafthttp/msgappv2_codec.go	69.56% <0%> (-1.74%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	72.48% <0%> (-1.35%)	⬇️
etcdserver/api/rafthttp/peer.go	78.77% <0%> (-1.12%)	⬇️
clientv3/lease.go	91.85% <0%> (-0.75%)	⬇️
pkg/proxy/server.go	60.2% <0%> (-0.68%)	⬇️
etcdserver/server.go	74.24% <0%> (-0.36%)	⬇️
lease/leasehttp/http.go	62.5% <0%> (ø)	⬆️
... and 12 more

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[xiang90]

lgtm