Update mapping documentation

Many mapping properties available in NHibernate are lacking proper
documentation. Document them. (Mainly done by synchronizing with
https://docs.jboss.org/hibernate/orm/3.5/reference/en-US/html/mapping.html
and subsequent chapters of the Hibernate documentation.)

Also fixes inaccuracies and misleading statements.

Author: fredericDelaporte

doc/reference/master.xml

@@ -8,6 +8,7 @@
 <!ENTITY persistent-classes     SYSTEM "modules/persistent_classes.xml">
 <!ENTITY basic-mapping          SYSTEM "modules/basic_mapping.xml">
 <!ENTITY collection-mapping     SYSTEM "modules/collection_mapping.xml">
+<!ENTITY associations           SYSTEM "modules/associations.xml">
 <!ENTITY component-mapping      SYSTEM "modules/component_mapping.xml">
 <!ENTITY inheritance-mapping    SYSTEM "modules/inheritance_mapping.xml">
 <!ENTITY manipulating-data      SYSTEM "modules/manipulating_data.xml">
@@ -55,6 +56,7 @@
 
 	&basic-mapping;
 	&collection-mapping;
+	&associations;
 	&component-mapping;
 	&inheritance-mapping;