Update storage sync docs to note that failed backups are not included

[a-mccarthy]

Signed-off-by: Abigail McCarthy [email protected]

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Please add a summary of your change

This updates the object storage syncing docs to include a message that failed or partially failed backups will not be removed by Velero.

Does your change fix a particular issue?

Fixes #4483

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov-commenter]

Codecov Report

Merging #4902 (183ead3) into main (db99b6e) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4902      +/-   ##
==========================================
- Coverage   41.70%   41.65%   -0.05%     
==========================================
  Files         204      204              
  Lines       18040    18040              
==========================================
- Hits         7523     7515       -8     
- Misses       9956     9963       +7     
- Partials      561      562       +1     

Impacted Files	Coverage Δ
pkg/restore/restore.go	66.02% <0.00%> (-0.65%)	⬇️

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[a-mccarthy]

closing in favor of #4729