docs: Document synchronization mutex usage.

[jmcarp]

I didn't realize argo supported synchronization without setting up a configmap--the mutex option is more convenient most of the time! I didn't see it in the docs, so I thought I would update the synchronization section.

h/t @maxhully for finding this

[codecov]

Codecov Report

Merging #7006 (6498fc7) into master (86ddda5) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7006      +/-   ##
==========================================
+ Coverage   48.48%   48.51%   +0.03%     
==========================================
  Files         265      265              
  Lines       19273    19273              
==========================================
+ Hits         9344     9350       +6     
+ Misses       8878     8870       -8     
- Partials     1051     1053       +2     

Impacted Files	Coverage Δ
server/workflow/workflow_server.go	46.80% <0.00%> (+1.06%)	⬆️
workflow/metrics/server.go	19.29% <0.00%> (+3.50%)	⬆️

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