Add DEFER clause (#1556)

Author: Dhghomon

src/content/doc-surrealql/statements/define/indexes.mdx

@@ -35,6 +35,7 @@ DEFINE INDEX [ OVERWRITE | IF NOT EXISTS ] @name
     [ @special_clause ]
     [ COMMENT @string ]
     [ CONCURRENTLY ]
+    [ DEFER ]
 ```
 
 The `@special_clause` part of the statement is an optional part in which an index can be declared for special usage such as guaranteeing unique values, full-text search, and so on. The available clauses are:
@@ -563,6 +564,21 @@ SELECT * FROM account WHERE name CONTAINS "Billy McConnell" EXPLAIN FULL;
 SELECT * FROM account WHERE name CONTAINSANY ["Billy McConnell"] EXPLAIN FULL;
 ```
 
+## The `DEFER` clause
+
+<Since v="v2.4.0" />
+
+Index updates in SurrealDB occur synchronously during document operations. This ensures **immediate consistency**, in which all reads return the most recent write. However, this can become a bottleneck during high-volume parallel ingestion, leading to write-write conflicts and increased latency, particularly with Full-Text or Vector indexes.
+
+The `DEFER` clause can be used in this case if **eventual consistency** is acceptable, namely a setting in which reads may return stale data for a short period, but will eventually converge to the most recent write. An index with this clause will be enqueued in a persistent background queue so that ingestion and indexing are decoupled.
+
+```surql
+DEFINE ANALYZER simple TOKENIZERS blank,class FILTERS lowercase;
+DEFINE INDEX title_index ON blog FIELDS title SEARCH ANALYZER simple BM25(1.2,0.75) HIGHLIGHTS DEFER;
+```
+
+Note: As unique indexes offer a guarantee that no records that contravene the index will ever exist, the `UNIQUE` clause cannot be used together with `DEFER`.
+
 ## Performance Implications
 
 When defining indexes, it's essential to consider the fields most frequently queried or used to optimize performance.