Clean up handling of attributes added through a SchemaSpec
Clean up handling of attributes added through a SchemaSpec

And document SchemaSpec properly

salzhrani/prosemirror,tzuhsiulin/prosemirror,mattberkowitz/prosemirror,kiejo/prosemirror,tgecho/prosemirror,jacwright/prosemirror,salzhrani/prosemirror,adrianheine/prosemirror,salzhrani/prosemirror,tzuhsiulin/prosemirror,bengler/prosemirror,tzuhsiulin/prosemirror,jacwright/prosemirror,kiejo/prosemirror,kiejo/prosemirror,adrianheine/prosemirror,ProseMirror/prosemirror-markdown,tgecho/prosemirror,adrianheine/prosemirror,mattberkowitz/prosemirror,bengler/prosemirror