Break up constrainst section to enhance readability

Author: treffynnon

_includes/sqlstyle.guide.md

@@ -298,7 +298,15 @@ changes to the definitions to keep them up to date.
 Once the keys are decided it is possible to define them in the system using
 constraints.
 
-* All tables must have at least one key to be useful.
+##### General
+
+* Tables must have at least one key to be complete and useful.
+* Constraints should be given a custom name excepting `UNIQUE`, `PRIMARY KEY`
+  and `FOREIGN KEY` where the database vendor will generally supply sufficiently
+  intelligible names automatically.
+
+##### Layout and order
+
 * Specify the primary key first right after the `CREATE TABLE` statement.
 * Constraints should be defined directly beneath the column they correspond to.
   Indent the constraint so that it aligns to the right of the column name.
@@ -307,20 +315,22 @@ constraints.
   include them at the end of the `CREATE TABLE` definition.
 * If it is a table level constraint that applies to the entire table then it
   should also appear at the end.
-* Use alphabetical order so `ON DELETE` comes before `ON UPDATE`.
-* All constraints should be given a custom name except `UNIQUE`, `PRIMARY KEY`
-  and `FOREIGN KEY` where the database vendor will generally supply sufficiently
-  intelligible names automatically.
+* Use alphabetical order where `ON DELETE` comes before `ON UPDATE`.
+* If it make sense to do so align each aspect of the query on the same character
+  position. For example all `NOT NULL` definitions should start at the same
+  character position.
+
+##### Validation
+
 * Use `LIKE` and `SIMILAR TO` constraints to ensure the integrity of strings
   where the format is known.
 * Where the ultimate range of a numerical value is known it must be written as a
   range `CHECK()` to prevent incorrect values entering the database or the silent
   truncation of data too large to fit the column definition. In the least it
   should check that the value is greater than zero in most cases.
 * `CHECK()` constraints should be kept in separate clauses to ease debugging.
-* If it make sense to do so align each aspect of the query on the same character
-  position. For example all `NOT NULL` definitions should start at the same
-  character position.
+
+##### Example
 
 ```sql
 CREATE TABLE staff (