docs, refactor docs about column modifiers. [ci skip] [Matthew Draper & Yves Senn]

This is a follow up to #15602 which rendered the guides in a weird state:

  >  You can also specify some options just after the field type between curly
     braces. You can use the following modifiers:

  >  `null`         Allows or disallows `NULL` values in the column.

  >  NOTE: `null` and `default` cannot be specified via command line.

The modifiers are now moved into a separate section. The generator
simply referes to that section.

Related to #15583.

/cc @JuanitoFatas

Conflicts:
	guides/source/migrations.md

guides/source/migrations.md

@@ -292,18 +292,10 @@ end
 
 You can append as many column name/type pairs as you want.
 
-### Supported Type Modifiers
+### Passing Modifiers
 
-You can also specify some options just after the field type between curly
-braces. You can use the following modifiers:
-
-* `limit`        Sets the maximum size of the `string/text/binary/integer` fields.
-* `precision`    Defines the precision for the `decimal` fields, representing the total number of digits in the number.
-* `scale`        Defines the scale for the `decimal` fields, representing the number of digits after the decimal point.
-* `polymorphic`  Adds a `type` column for `belongs_to` associations.
-* `null`         Allows or disallows `NULL` values in the column.
-
-NOTE: `null` and `default` cannot be specified via command line.
+Some commonly used [type modifiers](#column-modifiers) can be passed directly on
+the command line. They are enclosed by curly braces and follow the field type:
 
 For instance, running:
 
@@ -322,6 +314,8 @@ class AddDetailsToProducts < ActiveRecord::Migration
 end
 ```
 
+TIP: Have a look at the generators help output for further details.
+
 Writing a Migration
 -------------------
 
@@ -416,6 +410,21 @@ end
 removes the `description` and `name` columns, creates a `part_number` string
 column and adds an index on it. Finally it renames the `upccode` column.
 
+### Column Modifiers
+
+Column modifiers can be applied when creating or changing a column:
+
+* `limit`        Sets the maximum size of the `string/text/binary/integer` fields.
+* `precision`    Defines the precision for the `decimal` fields, representing the total number of digits in the number.
+* `scale`        Defines the scale for the `decimal` fields, representing the number of digits after the decimal point.
+* `polymorphic`  Adds a `type` column for `belongs_to` associations.
+* `null`         Allows or disallows `NULL` values in the column.
+* `default`      Allows to set a default value on the column. NOTE: If using a dynamic value (such as date), the default will only be calculated the first time (e.g. on the date the migration is applied.)
+* `index`        Adds an index for the column.
+
+Some adapters may support additional options; see the adapter specific API docs
+for further information.
+
 ### When Helpers aren't Enough
 
 If the helpers provided by Active Record aren't enough you can use the `execute`