chore(doc): note that filesync works for debug

[briandealwis]

Description
File sync does work with debug, but is disabled by default.

We should document some of the limitations of sync since some users assume that it magically rebuilds their applications. Some context in https://chat.google.com/room/AAAAuTCR_1s/srcFk70Ok_A

[codecov]

Codecov Report

Merging #8044 (3aa1eb6) into main (290280e) will decrease coverage by 3.93%.
The diff coverage is 54.09%.

@@            Coverage Diff             @@
##             main    #8044      +/-   ##
==========================================
- Coverage   70.48%   66.54%   -3.94%     
==========================================
  Files         515      598      +83     
  Lines       23150    29033    +5883     
==========================================
+ Hits        16317    19321    +3004     
- Misses       5776     8283    +2507     
- Partials     1057     1429     +372     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/credits/export.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/deploy.go	40.90% <0.00%> (-12.94%)	⬇️
cmd/skaffold/app/cmd/test.go	44.44% <0.00%> (ø)
cmd/skaffold/app/exitcode.go	100.00% <ø> (+6.66%)	⬆️
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/inspect_tests.go	62.50% <14.28%> (-1.14%)	⬇️
cmd/skaffold/app/cmd/render.go	35.48% <18.18%> (-5.90%)	⬇️
cmd/skaffold/app/cmd/lsp.go	28.12% <28.12%> (ø)
cmd/skaffold/app/cmd/run.go	64.28% <33.33%> (-9.63%)	⬇️
cmd/skaffold/app/cmd/fix.go	56.41% <37.50%> (-20.07%)	⬇️
... and 394 more

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[aaron-prindle]

LGTM!