Docs: Clarify Spark branch write precedence over WAP branch

[manuzhang]

Feature Request / Improvement

After #15288, Spark Writing to Branches doc (docs/docs/spark-writes.md) no longer matches that in codes.

   * <p>The target branch can be specified via table identifier, write option, or in SQL:
   *
   * <ul>
   *   <li>The identifier and option branches can't conflict. If both are set, they must match.
   *   <li>Identifier and option branches take priority over the session WAP branch.
   *   <li>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.
   * </ul>
   *
   * <p>Note: WAP ID and WAP branch cannot be set at the same time.
   *
   * <p>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

[yadavay-amzn]

I'd like to work on this. I've submitted a fix in #15917 that updates the docs to reflect the new branch precedence behavior from #15288.