Document that index names are made up of all columns, not just the first.

index_name has been using the following expression

    "index_#{table_name}_on_#{Array.wrap(options[:column]) * '_and_'}"

since at least 2006 (bc7f2315), and that's how they come out in my DB.

Please check that this is correct before merging into master, perhaps
I'm misunderstanding the section I changed.

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

@@ -301,15 +301,8 @@ module ActiveRecord
       # Adds a new index to the table. +column_name+ can be a single Symbol, or
       # an Array of Symbols.
       #
-      # The index will be named after the table and the first column name,
-      # unless you pass <tt>:name</tt> as an option.
-      #
-      # When creating an index on multiple columns, the first column is used as a name
-      # for the index. For example, when you specify an index on two columns
-      # [<tt>:first</tt>, <tt>:last</tt>], the DBMS creates an index for both columns as well as an
-      # index for the first column <tt>:first</tt>. Using just the first name for this index
-      # makes sense, because you will never have to create a singular index with this
-      # name.
+      # The index will be named after the table and the column name(s), unless
+      # you pass <tt>:name</tt> as an option.
       #
       # ===== Examples
       #