Docs: Clarify Spark branch write precedence over WAP branch

### Feature Request / Improvement

After https://github.com/apache/iceberg/pull/15288, [Spark Writing to Branches doc](https://iceberg.apache.org/docs/nightly/spark-writes/#writing-to-branches) (`docs/docs/spark-writes.md`) no longer matches that in codes.

```
*

The target branch can be specified via table identifier, write option, or in SQL:
*
*

*
• The identifier and option branches can't conflict. If both are set, they must match.
  *
• Identifier and option branches take priority over the session WAP branch.
  *
• If neither the option nor the identifier branch is set and WAP is enabled for this table,
  * use the WAP branch from the session SQL config.
  *

Note: WAP ID and WAP branch cannot be set at the same time.
*
*

Note: The target branch may be created during the write operation if it does not exist.
```

### Query engine

Spark

### Willingness to contribute

- [ ] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start with the Writing to Branches section in docs/docs/spark-writes.md and compare it with the precedence described in the issue. Update the documentation so identifier and write-option branches are documented as taking priority over the session WAP branch, and ensure the listed fallback behavior matches the referenced code.

Written by the indexing model from the issue text.