Fix doc of limit option for a text column [ci skip]

Follow up #21591.

The document of limit option for a text column is incorrect.

MySQL: the limit is byte length, not character length
Pg, Sqlite3: variable unlimited length

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

@@ -236,8 +236,8 @@ def [](name)
       #
       # Available options are (none of these exists by default):
       # * <tt>:limit</tt> -
-      #   Requests a maximum column length. This is number of characters for <tt>:string</tt> and
-      #   <tt>:text</tt> columns and number of bytes for <tt>:binary</tt> and <tt>:integer</tt> columns.
+      #   Requests a maximum column length. This is number of characters for a <tt>:string</tt> column
+      #   and number of bytes for <tt>:text</tt>, <tt>:binary</tt> and <tt>:integer</tt> columns.
       # * <tt>:default</tt> -
       #   The column's default value. Use nil for NULL.
       # * <tt>:null</tt> -