Fixes 730: document inter-router protocol

[mgoulish]

Documenting how a router reacts to a new receiving client, and communicates the new address to other routers.

[codecov]

Codecov Report

Merging #1387 (b64d225) into main (93abd7c) will increase coverage by 0.0%.
The diff coverage is n/a.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1387   +/-   ##
=====================================
  Coverage   77.7%   77.7%           
=====================================
  Files        247     247           
  Lines      64084   64084           
  Branches    5905    5905           
=====================================
+ Hits       49804   49812    +8     
+ Misses     11556   11546   -10     
- Partials    2724    2726    +2     

Flag	Coverage Δ
pysystemtests	87.6% <ø> (-0.1%)	⬇️
pyunittests	54.5% <ø> (ø)
systemtests	71.3% <ø> (+<0.1%)	⬆️
unittests	21.8% <ø> (ø)

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Components	Coverage Δ
unittests	25.9% <ø> (ø)
systemtests	78.4% <ø> (+<0.1%)	⬆️