atkdocument: deprecation of some methods

atk_document_get_document, atk_document_get_document_type and their
equivalent virtuals were deprecated. Documentation properly updated.

At least atk_document_get_document_type seems to be announced as
deprecated since a long time ago. At least firefox code (in form
of comments) mention that. The case of atk_document_get_document
seems worse, as it doesn't makes too much sense. None of them
are used at at-spi at all.
doc: improve documentation of AtkAttribute and AtkAttributeSet