Introduction: VFS documentation and tidy up

The following 4 patches improve the documentation for vfs address_space
operations, and clean up some code oddities I found while writing it.

All have been sent the these lists before for review and only one
comment was received: a positive note about the documentation.

Please target them for 2.6.17.

Thanks,
NeilBrown


 [PATCH 001 of 4] Update some VFS documentation.
 [PATCH 002 of 4] Honour AOP_TRUNCATE_PAGE returns in page_symlink
 [PATCH 003 of 4] Make address_space_operations->sync_page return void.
 [PATCH 004 of 4] Make address_space_operations->invalidatepage return void