#4098 Docs: document clearDirty on the datastore sync service and the SWAMP_V3_RESTORE_PROBE_MS variable

What changed

swamp-club#4091 and swamp-club#4092 shipped two things that the manual under content/manual/reference/ does not describe yet.

Pages to update

1. extensions/datastore-provider.md: the sync-service method table lists markDirty, hydrateFile and fetchContent. Add a row:

   | clearDirty | (relPaths: readonly string[]) => Promise<void> | No | Drop the per-path marks whose keys equal relPaths, made before the call. Called after swamp undoes a refused push's writes. |

   Add a short paragraph below the markDirty section: exact keys only, marks made during or after the call are kept, bulk and overflow flags stay, nothing remote is touched, and swamp treats the method's presence as the capability. Point to the clear-dirty conformance case.

2. datastore-configuration.md: document SWAMP_V3_RESTORE_PROBE_MS, a positive integer of milliseconds (default 4000) that bounds each listing swamp makes of a v3 replica while a restore runs. Raise it for a replica whose first listing is always slow (a far region, a slow S3-compatible store, a cold bucket); otherwise every lagging read on that checkout fails with datastore_v3_catch_up_unreachable.