Hint at advanced options for foreign_key

We sometimes display simple examples of additional parameters that can be
supplied to table-wise methods like these and I found it particularly difficult
to figure out which options `t.foreign_key` accepts without drilling very deep
into the specific SchemaStatements docs.

Since it's relatively common to create foreign keys with custom column names or
primary keys, it seems like this should help quite a few people.

[ci skip]

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -680,9 +680,10 @@ def remove_references(*args, **options)
       end
       alias :remove_belongs_to :remove_references
 
-      # Adds a foreign key.
+      # Adds a foreign key to the table using a supplied table name.
       #
       #  t.foreign_key(:authors)
+      #  t.foreign_key(:authors, column: :author_id, primary_key: "id")
       #
       # See {connection.add_foreign_key}[rdoc-ref:SchemaStatements#add_foreign_key]
       def foreign_key(*args)