Seed: improve helptext

[arajasek]

Also change to a Uint since that's what we parse in the body

[codecov]

Codecov Report

Merging #7304 (16c6091) into master (d1d8845) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7304      +/-   ##
==========================================
- Coverage   39.12%   39.10%   -0.02%     
==========================================
  Files         614      614              
  Lines       64983    64983              
==========================================
- Hits        25422    25414       -8     
- Misses      35155    35156       +1     
- Partials     4406     4413       +7     

Impacted Files	Coverage Δ
cmd/lotus-seed/main.go	0.00% <ø> (ø)
chain/events/message_cache.go	87.50% <0.00%> (-12.50%)	⬇️
chain/types/tipset_key.go	85.96% <0.00%> (-3.51%)	⬇️
node/hello/hello.go	63.63% <0.00%> (-3.41%)	⬇️
chain/sub/incoming.go	54.43% <0.00%> (-3.38%)	⬇️
storage/wdpost_sched.go	77.22% <0.00%> (-1.99%)	⬇️
chain/events/events_called.go	83.90% <0.00%> (-1.96%)	⬇️
extern/sector-storage/sched.go	83.05% <0.00%> (-1.66%)	⬇️
chain/stmgr/searchwait.go	63.39% <0.00%> (-1.31%)	⬇️
storage/wdpost_changehandler.go	97.64% <0.00%> (-0.95%)	⬇️
... and 11 more

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