Update datefield.md

Removed ; at the end of ->setConfig('dmyfields', true) in the three chain example. Is confusing for new users/developers who are not aware of how to chain methods.

Closes #3006
Update dataobject.md

This page is grid-field not gridfield.
Merge pull request #3009 from scottmcmurray/patch-1

Update dataobject.md
Prevent IE errors on hidden login forms

In order to focus a field, it needs to be visible,
which can't be guaranteed on a core level by the login form JavaScript.
Optionally check for visibility via jQuery if it exists,
and allow explicit disabling of this behaviour via a unique identifier.