Docs: CHAR(n) trailing-space behavior is misleading and CDC impact is undocumented

## Summary

The current documentation for `CHAR(n)` / `CHARACTER(n)` is misleading about how trailing spaces are handled, and there is no mention of the significant impact this has on CDC / Changefeeds.

---

## Current (misleading) docs statement

> For `CHAR(n)`/`CHARACTER(n)` types, if the value is under the column's length limit, CockroachDB adds space padding from the end of the value to the length limit.

This implies that a value like `' '` (one space) stored in a `CHAR(3)` column would be read back as `' '` (three spaces). That is **not** what happens in SQL expressions.

---

## Actual behavior (matches PostgreSQL)

Trailing spaces are treated as semantically insignificant in `CHAR`/`BPCHAR` types, per the SQL standard and PostgreSQL behavior:

- Trailing spaces are **stripped** when a value is stored into a `CHAR` column (CRDB: [`datum.go#L6283-L6285`](https://github.com/cockroachdb/cockroach/blob/3e041788939c2cfec43af8388d7b9524b85ea779/pkg/sql/sem/tree/datum.go#L6283-L6285), [`cast.go#L540-L543`](https://github.com/cockroachdb/cockroach/blob/47b117c1c010ee4f61ce7a551cb06ce5b1108141/pkg/sql/sem/eval/cast.go#L540-L543)).
- Space padding **is** re-added when values are returned to the client over pgwire ([`types.go#L105-L114`](https://github.com/cockroachdb/cockroach/blob/47b117c1c010ee4f61ce7a551cb06ce5b1108141/pkg/sql/pgwire/types.go#L105-L114)), so `SELECT v FROM fab` shows `' '` in a psql client — but SQL expressions like `length(v)` operate on the stripped value.

### Demonstration (CockroachDB v25.4.5 and v24.1.25, matches PostgreSQL 18.3):

```sql
CREATE TABLE fab (id INT NOT NULL PRIMARY KEY, v CHAR(3));

INSERT INTO fab VALUES (1, NULL), (2, ''), (3, ' '), (4, ' '),
(5, 'abc'), (6, 'a '), (7, ' b '), (8, ' c');

SELECT *, length(v) FROM fab;
-- id | v | length
-- ----+------+--------
-- 1 | NULL | NULL
-- 2 | | 0
-- 3 | | 0 -- trailing space stripped
-- 4 | | 0 -- all spaces stripped
-- 5 | abc | 3
-- 6 | a | 1 -- trailing spaces stripped
-- 7 | b | 2 -- trailing space stripped
-- 8 | c | 3
```

Compare with `STRING(3)` / `VARCHAR(3)`, where trailing spaces **are** preserved:

```sql
-- 3 | | 1 -- space preserved
-- 4 | | 3 -- spaces preserved
-- 6 | a | 3
-- 7 | b | 3
```

This matches the PostgreSQL docs:
> Trailing spaces are treated as semantically insignificant and disregarded when comparing two values of type `character`. Trailing spaces are removed when converting a `character` value to one of the other string types.

---

## CDC / Changefeed impact (undocumented)

This behavior causes **data loss in CDC exports** because the changefeed reads values via SQL, not pgwire padding. A `CHAR(3)` column with value `' '` (one space) is exported as an empty string:

```
$ cat changefeed_output.csv | sort
1,NULL
2,
3, # ← was ' ', exported as empty
4, # ← was ' ', exported as empty
5,abc
6,a # ← was 'a ', trailing spaces lost
7," b"
8," c"
```

This is particularly impactful for customers with large datasets (e.g., hundreds of millions of rows) using `CHAR` columns expecting space-preservation in exports.

---

## Recommended workaround for CDC

Use `rpad()` in a changefeed `SELECT` projection to explicitly restore padding:

```sql
CREATE CHANGEFEED INTO ''
WITH initial_scan = 'only', format = csv
AS SELECT id, rpad(v, 3) FROM fab;
```

Output:
```
1,NULL
2," "
3," "
4," "
5,abc
6,a
7," b "
8," c"
```

---

## Suggested doc improvements

1. **Clarify the `CHAR(n)` description**: Explain that trailing spaces are stripped at storage time and re-added only when values are returned via the wire protocol — they are NOT present in SQL expression evaluation (e.g., `length()`, comparisons, casts).
2. **Add a note on `CHAR` vs `STRING`/`VARCHAR`**: If trailing space preservation is required, `STRING(n)` / `VARCHAR(n)` should be used instead.
3. **Add a CDC/Changefeed note**: Document that `CHAR(n)` columns will have trailing spaces stripped in changefeed CSV/Avro exports, and recommend using `rpad(v, n)` in a changefeed `SELECT` projection as a workaround.

---

## References

- Internal discussion: Slack `#help-database`, 2026-05-15
- PostgreSQL docs: https://www.postgresql.org/docs/current/datatype-character.html
- Affected versions confirmed: v24.1.25, v25.4.5

Jira issue: DOC-17107