Update secrets field API doc

[liggitt]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Fixes the explanation on the ServiceAccount secrets field about how pod enforcement works, and clarifies this field should not be used for scraping generated token secrets.

NONE

/sig auth
/assign @zshihang @smarterclayton

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[zshihang]

/lgtm

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/retest