Rename broadcast validation to broadcast mode and add docs

[avalonche]

📝 Summary

⛱ Motivation and Context

📚 References

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md

[codecov-commenter]

Codecov Report

Merging #506 (0e01140) into main (7df7bdd) will increase coverage by 0.02%.
Report is 1 commits behind head on main.
The diff coverage is 13.33%.

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@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
+ Coverage   31.68%   31.70%   +0.02%     
==========================================
  Files          24       24              
  Lines        4860     4860              
==========================================
+ Hits         1540     1541       +1     
+ Misses       3128     3127       -1     
  Partials      192      192              

Flag	Coverage Δ
unittests	31.70% <13.33%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
beaconclient/mock_beacon_instance.go	63.15% <0.00%> (ø)
beaconclient/prod_beacon_instance.go	12.59% <0.00%> (ø)
beaconclient/util.go	24.59% <0.00%> (ø)
beaconclient/multi_beacon_client.go	41.07% <22.22%> (+0.41%)	⬆️

[jtraglia]

LGTM just one nit.

[jtraglia]

LGTM