explain-joins: reword "create ... on disk" as "spill ... to disk"

In the hash join section, one sentence said TiDB "might create the `Build`
operator ... on disk", which is confusing. Align it with the wording used
later in the same page ("spill the `Build` operator ... to disk"), which more
accurately describes spilling the operator's data to temporary storage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: yahonda

explain-joins.md

@@ -219,7 +219,7 @@ The `operator info` column in the `EXPLAIN` result table also records other info
 
 ### Runtime Statistics
 
-If [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query) (default value: 1 GB) is exceeded, and the [`tidb_enable_tmp_storage_on_oom`](/system-variables.md#tidb_enable_tmp_storage_on_oom) value is `ON` (default), TiDB will attempt to use temporary storage, and might create the `Build` operator (used as part of the hash join) on disk. Runtime statistics such as memory usage are recorded in the `execution info` of the `EXPLAIN ANALYZE` result table. The following example shows the output of `EXPLAIN ANALYZE` with a 1 GB (default) and a 500 MB quota for `tidb_mem_quota_query`. At 500 MB, disk is used for temporary storage:
+If [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query) (default value: 1 GB) is exceeded, and the [`tidb_enable_tmp_storage_on_oom`](/system-variables.md#tidb_enable_tmp_storage_on_oom) value is `ON` (default), TiDB will attempt to use temporary storage, and might spill the `Build` operator (used as part of the hash join) to disk. Runtime statistics such as memory usage are recorded in the `execution info` of the `EXPLAIN ANALYZE` result table. The following example shows the output of `EXPLAIN ANALYZE` with a 1 GB (default) and a 500 MB quota for `tidb_mem_quota_query`. At 500 MB, disk is used for temporary storage:
 
 ```sql
 EXPLAIN ANALYZE SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;