Add column modifiers docs for PostgreSQL spatial column type (#7438)

* Add column modifiers docs for PostgreSQL spatial column type

* Update migrations.md

Co-authored-by: Taylor Otwell <[email protected]>

Author: aanfarhan

migrations.md

@@ -875,6 +875,7 @@ Modifier  |  Description
 `->virtualAs($expression)`  |  Create a virtual generated column (MySQL).
 `->generatedAs($expression)`  |  Create an identity column with specified sequence options (PostgreSQL).
 `->always()`  |  Defines the precedence of sequence values over input for an identity column (PostgreSQL).
+`->isGeometry()`  |  Set spatial column type to `geometry` - the default type is `geography` (PostgreSQL).
 
 <a name="default-expressions"></a>
 #### Default Expressions