Add documentation for fastcgi_finish_request() (and add an FPM book as a
whole).

This is the first part of a rainy day task to try to clean up some of our build
warnings.


git-svn-id: https://svn.php.net/repository/phpdoc/doc-base/trunk@331257 c90b9560-bf6c-de11-be94-00142212c4b1