Add envconfig docs to the config

[magik6k]

No description provided.

[codecov]

Codecov Report

Merging #7412 (b3432b5) into master (4192e40) will increase coverage by 5.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7412      +/-   ##
==========================================
+ Coverage   34.44%   39.62%   +5.17%     
==========================================
  Files         609      615       +6     
  Lines       64851    65140     +289     
==========================================
+ Hits        22341    25809    +3468     
+ Misses      38314    34843    -3471     
- Partials     4196     4488     +292     

Impacted Files	Coverage Δ
node/config/load.go	69.13% <100.00%> (+0.38%)	⬆️
markets/loggers/loggers.go	89.28% <0.00%> (-10.72%)	⬇️
chain/actors/builtin/paych/paych.go	16.88% <0.00%> (-5.20%)	⬇️
chain/events/observer.go	71.64% <0.00%> (-2.99%)	⬇️
miner/miner.go	55.29% <0.00%> (-0.67%)	⬇️
node/impl/full/chain.go	12.34% <0.00%> (-0.61%)	⬇️
node/impl/storminer.go	27.03% <0.00%> (-0.40%)	⬇️
extern/sector-storage/stores/local.go	58.80% <0.00%> (ø)
journal/mockjournal/journal.go	71.42% <0.00%> (ø)
lib/nullreader/reader.go	0.00% <0.00%> (ø)
... and 137 more

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