proto: add a comment for Validator#Address

[melekes]

Refs #3732

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[codecov]

Codecov Report

Merging #5144 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5144      +/-   ##
==========================================
+ Coverage   62.46%   62.52%   +0.06%     
==========================================
  Files         259      259              
  Lines       27279    27279              
==========================================
+ Hits        17040    17057      +17     
+ Misses       8750     8737      -13     
+ Partials     1489     1485       -4     

Impacted Files	Coverage Δ
types/validator.go	81.91% <ø> (ø)
privval/signer_server.go	91.11% <0.00%> (-4.45%)	⬇️
consensus/reactor.go	75.34% <0.00%> (-1.28%)	⬇️
statesync/syncer.go	79.44% <0.00%> (-0.80%)	⬇️
blockchain/v0/pool.go	78.66% <0.00%> (-0.64%)	⬇️
p2p/pex/addrbook.go	73.36% <0.00%> (+0.87%)	⬆️
blockchain/v0/reactor.go	64.18% <0.00%> (+0.93%)	⬆️
consensus/state.go	73.06% <0.00%> (+1.28%)	⬆️
libs/clist/clist.go	68.18% <0.00%> (+1.51%)	⬆️
p2p/pex/pex_reactor.go	82.53% <0.00%> (+1.58%)	⬆️
... and 2 more