docs: sync config.sample.php parameters for 10.15

[oc-tmueller]

Same staleness #138 just fixed for 10.16, on the 10.15 line. web-updater.enabled was added to core's config/config.sample.php in owncloud/core#41385 — present on the 10.15 branch too — and this page was never regenerated.

Regenerated with config-to-docs from core 10.15, not hand-written. The added block is byte-identical to the one #138 added for 10.16, which is expected: config.sample.php is itself identical between origin/10.15 and origin/10.16, and after this the two published pages are byte-identical too.

Why this completes something

With this merged, all three published server versions regenerate to an empty diff against their core branch:

docs	core branch	keys	drift
content/server/11.0	master	159/159	none
content/server/10.16	10.16	159/159	none (after #138)
content/server/10.15	10.15	159/159	none (this PR)

Verified by regenerating all four generated pages and diffing, and separately by comparing core's full top-level key set against each page's sections — no key missing, none extra, order identical. 11.0 correctly lacks web-updater.enabled: the whole updater/ directory is gone on master.

That empty-diff state is the precondition for checking it in CI rather than discovering drift years later, which is the follow-up.

Two core-side issues this surfaced, neither fixed here

Both are in core's comment text, so fixing them in generated output would re-break the invariant above.

1. A contradiction, as flagged on #138. A few lines above the insertion point, upgrade.disable-web still says "The web based updater is enabled by default" — invalidated by the very commit that added the parameter being synced here.

2. Published documentation is currently losing information. Core writes:

 * 0 -> Never check the filesystem for outside changes, provides a performance
 * 1 -> Check each file or folder at least once per request

and all three published pages render:

-> Never check the filesystem for outside changes, provides a performance

The 0 and 1 are gone, so a reader cannot tell which value does which. Cause: the converter strips a line's leading run when it is a single character followed by - — a filter that exists to pull indented - lists flush left. It is now documented in config-to-docs#31, and this is a live instance of it. Fix is a one-line rewording in core (e.g. backtick the values, or use 0: instead of 0 ->).

Checks

npm ci && npm run antora && npm test pass (51 tests); the new section renders on the built 10.15 page. Region above the // header end marker byte-identical to main.

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