doc: deprecation clarifications
Clarify some details on the deprecation policy. Specifically:
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Clarify that deprecated code is not subject to the typical support (current or LTS) policies
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Clarify that deprecation does not automatically mean removal. Code may be docs-deprecated or runtime deprecated indefinitely.
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Clarify that End-of-Life does not necessarily mean removal. What it means is that changes are no longer subject to the semver rules, up to and including removal.
Checklist
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documentation is changed or added -
commit message follows commit guidelines