docs(durability): document the new-bucket relaxed default (#1811)
#5971 seeds a `relaxed` durability override into newly created buckets'
metadata (rustfs/backlog#1811), but the operator guide still described the
default as `strict` with relaxed as opt-in, and never mentioned the new-bucket
seed or its `RUSTFS_NEW_BUCKET_DURABILITY_MODE` knob. Document the behavior:
- intro notes new buckets default to `relaxed` (gradual migration; the
process-wide default and pre-existing buckets stay `strict`);
- the Configuration block lists `RUSTFS_NEW_BUCKET_DURABILITY_MODE`
(relaxed|strict|none|inherit, default relaxed);
- a new "New-bucket default" subsection under per-bucket durability covers the
seed semantics, the opt-out (`inherit` / `=strict`), fail-closed invalid
values, and that existing buckets are never retroactively rewritten.
No code change.
Co-authored-by: heihutu <[email protected]>
