Document policy regarding backports of dependency updates

[spiffxp]

Prompted by kubernetes/kubernetes#94367 getting cherry picked with no explicit documentation as to why

From this slack thread with @justaugustus

We commonly backport deps in the same minor stream
[The policy is] handwavy, needs to be written this cycle
The plan is to extend out from this Golang handbook (merged PR): #1109

/sig release
/area release-eng
/kind documentation

[justaugustus]

/assign
/priority important-soon
/area dependency

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