rest api: update WaitForBlock summary

[algorandskiy]

Summary

WaitForBlock waits for r+1 so updated the method summary.
Resolves https://github.com/algorand/go-algorand-internal/issues/1361

Test Plan

Existing tests

[codecov]

Codecov Report

Merging #4910 (7d6c3fc) into master (c8121b0) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4910      +/-   ##
==========================================
+ Coverage   54.08%   54.11%   +0.03%     
==========================================
  Files         427      427              
  Lines       53528    53528              
==========================================
+ Hits        28948    28965      +17     
+ Misses      22314    22300      -14     
+ Partials     2266     2263       -3     

Impacted Files	Coverage Δ
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
ledger/testing/randomAccounts.go	57.23% <0.00%> (+0.61%)	⬆️
ledger/tracker.go	78.90% <0.00%> (+0.84%)	⬆️
catchup/service.go	69.80% <0.00%> (+1.20%)	⬆️
network/wsPeer.go	71.36% <0.00%> (+2.38%)	⬆️

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