docs: document .torchxconfig usage with component:

[d4l3k]

This documents .torchxconfig [component:foo] defaults.

Test plan:

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Codecov Report

Merging #434 (bcf455f) into main (434c013) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #434   +/-   ##
=======================================
  Coverage   94.52%   94.52%           
=======================================
  Files          64       64           
  Lines        3782     3782           
=======================================
  Hits         3575     3575           
  Misses        207      207           

Impacted Files	Coverage Δ
torchx/runner/config.py	100.00% <ø> (ø)

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