Fix 64624 document symbolic mode limitation

[mu-lippy]

What does this PR do?

Adds documentation notes to clarify that Salt's file module only supports numeric mode specifications (like 644) and does not support symbolic modes (like u+rw) that are commonly used with chmod.

What issues does this PR fix or reference?

Fixes #64624

Merge requirements satisfied?

• [ x] Docs
• [ x] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html

Commits signed with GPG?

No

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