Document multi-stage dockerfile limitations of filesync

[briandealwis]

Fixes: #6525

Description
Document the limitations of the infer sync rules with multi-stage dockerfiles.

[codecov]

Codecov Report

Merging #6526 (c7e1ed6) into main (997edc8) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6526      +/-   ##
==========================================
- Coverage   70.43%   70.41%   -0.02%     
==========================================
  Files         515      515              
  Lines       23127    23127              
==========================================
- Hits        16289    16286       -3     
- Misses       5780     5783       +3     
  Partials     1058     1058              

Impacted Files	Coverage Δ
pkg/skaffold/log/stream/stream.go	71.42% <0.00%> (-14.29%)	⬇️

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[aaron-prindle]

LGTM