corrected English grammar in docblocks
Update override field mapping tut

* Added reference to override section of inheritance-mapping.rst
* Added link to yaml/xml examples in override-field-association-mappings-in-subclasses.rst
Merge pull request #755 from shieldo/patch-2

corrected English grammar in docblocks
Merge pull request #756 from peterjmit/master

Make yaml/xml configuration for  "AttributeOverride" and "AssociationOverride" more discoverable