Add docs on ForNpgsqlHasIndexMethod

(cherry picked from commit e678c24)

Author: roji

doc/misc.md

@@ -23,6 +23,19 @@ To enable this feature on an entity, insert the following code into your model's
 modelBuilder.Entity<MyEntity>().ForNpgsqlUseXminAsConcurrencyToken();
 ```
 
+## PostgreSQL Index Methods
+
+PostgreSQL supports a number of _index methods_, or _types_. These are specified at index creation time via the `USING _method_` clause, see the [PostgreSQL docs for `CREATE INDEX`](https://www.postgresql.org/docs/current/static/sql-createindex.html) and [this page](https://www.postgresql.org/docs/current/static/indexes-types.html) for info on the different types.
+
+The Npgsql EF Core provider allows you to specify the index method to be used by specifying `ForNpgsqlHasMethod()` on your index in your context's OnModelCreating:
+```c#
+protected override void OnModelCreating(ModelBuilder modelBuilder) {
+    modelBuilder.Entity<Blog>()
+        .HasIndex(b => b.Url)
+        .ForNpgsqlHasMethod("gin");
+}
+```
+
 ## Execution Strategy
 
 Since 2.0.0, the Npgsql provider provides a retrying execution strategy, which will attempt to detect most transient PostgreSQL/network errors and will automatically retry your operation. To enable, place the following code in your context's `OnModelConfiguring`:
@@ -67,7 +80,7 @@ which will be copied as the basis for the new one. This can be useful for includ
 modelBuilder.HasDatabaseTemplate("my_template_db");
 ```
 
-# CockroachDB Interleave In Parent
+## CockroachDB Interleave In Parent
 
 If you're using CockroachDB, the Npgsql provider exposes its ["interleave in parent" feature](https://www.cockroachlabs.com/docs/stable/interleave-in-parent.html). Use the following code: