Update zend.mail.attachments.rst

Rewrote the whole page since it was just a copy of ZF1 docs with namespaces.
correct some class names
Merge branch 'hotfix/#881'

Close #881
Merge branch 'patch-1' of https://github.com/stefanotorresi/zf2-documentation into hotfix/#875
Merge branch 'hotfix/#875' into develop

Close #875