docs: describe what APP_CACHE_BUSTER actually does

[anurag6569201]

What does this PR do?

The release checklist said to increment APP_CACHE_BUSTER by 1 on every patch, and listed it alongside APP_VERSION_STABLE as metadata that must match the target version. It is not a version number and does not track releases.

Its only consumer is the md5 in Request::cacheIdentifier(), which keys the server-side response cache for the seven routes labelled cache — file preview and the avatars endpoints. Changing it changes every key at once, so every entry is orphaned and every image transform and screenshot regenerates. That is worth doing only when the cached bytes would now be wrong.

Practice already disagreed with the docs: 1.9.6 → 2.0.0 deliberately kept 4327 in 98f425f, because 2.0 changes no cached response shape. This makes the docs match.

Docs only, no behaviour change.

Source merge-base: 7da1cdb08916a4b6a86e5e23ad52c3a5f0668a2b
Source head: c05099ff03ebc7bb6e4858905a4ab72dc24b0bcd

[shipwright-agent]

✅ Shipwright · Approve

Recommendation: approve PR #13 · Tier T0
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Next step: ready to merge.

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