Add documentation for :collation column option (#33733)

* Add documentation for `:collation` option

The table definition supports a `:collation` option for string and text columns, but this is not documented anywhere that I could find.

I'm not sure if the "If not specified" part is accurate. From [this PR](1515c4d), it looks like it passes `nil` and lets the database handle the collation, but I'm happy to change it if I misread the code.

[ci skip]

* FIX remove whitespace

[Nate Pinsky + Rafael Mendonça França]

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

@@ -522,6 +522,9 @@ def drop_table(table_name, options = {})
       #   Specifies the precision for the <tt>:decimal</tt> and <tt>:numeric</tt> columns.
       # * <tt>:scale</tt> -
       #   Specifies the scale for the <tt>:decimal</tt> and <tt>:numeric</tt> columns.
+      # * <tt>:collation</tt> -
+      #   Specifies the collation for a <tt>:string</tt> or <tt>:text</tt> column. If not specified, the
+      #   column will have the same collation as the table.
       # * <tt>:comment</tt> -
       #   Specifies the comment for the column. This option is ignored by some backends.
       #