docs: Complete the scoped freshness CCDF query

[maheshwarip]

Motivation

On How to monitor freshness, the last code block in the "Summarize freshness with a CCDF" section is incomplete. It shows only the middle of the statement (the object-name placeholder below is spelled MV_NAME here, and appears in the page as that name wrapped in angle brackets):

    FROM mz_internal.mz_wallclock_global_lag_recent_history wl
    JOIN mz_catalog.mz_objects o ON wl.object_id = o.id
    WHERE o.name = 'MV_NAME'
      AND wl.lag IS NOT NULL
      AND wl.lag > INTERVAL '0'

It starts mid-statement at FROM, drops the SELECT extract(epoch FROM wl.lag) AS lag_seconds line that the FROM belongs to, and has no enclosing WITH lags AS (, no thresholds CTE, and no outer aggregation. Running it verbatim fails:

ERROR:  Unexpected keyword FROM at the beginning of a statement
LINE 1: FROM mz_internal.mz_wallclock_global_lag_recent_history wl
        ^

A reader has to reassemble the statement by hand from the aggregate query further up the page. Every other code block on this page is a complete, runnable statement, so this one is the odd one out.

Description

Show the whole statement in the block, so it can be copied and run directly, and reword the lead-in sentence so it describes the query that follows rather than reading as a patch instruction ("add a join ... to the lags CTE" becomes "join mz_catalog.mz_objects in the lags CTE and filter on the object name").

The SQL is unchanged from what the fragment implied: the lags CTE gains the join to mz_catalog.mz_objects and the object-name filter, and the rest of the query matches the aggregate version above it. mz_wallclock_global_lag_recent_history.object_id is documented as corresponding to mz_objects.id, so the join key is correct. The placeholder in the page is left exactly as it was.

Verification

Manual, against a local environmentd built from this branch with a materialized view producing wallclock lag history.

• The block as it appears on the published page fails with ERROR: Unexpected keyword FROM at the beginning of a statement (exit 3).
• The block extracted programmatically from the edited markdown runs clean and returns the three-row shape the surrounding prose describes:

 lag_threshold_seconds | fraction_of_time_at_or_above
-----------------------+------------------------------
                     1 |                            1
                    10 |                            0
                   100 |                            0
(3 rows)

• Re-ran the page's other two mzsql blocks (the per-object lag history query and the aggregate CCDF) to confirm they were already complete and still run clean.

Docs-only change, so no automated tests were added or modified.

Release notes

No release note. This is a documentation-only fix with no user-visible product change.

[maheshwarip]

@sjwiesman could I get a quick 👍🏾 on this one?