Remove outdated section about one-to-many relations between viewpoints and comments

This section in the documentation is outdated: https://github.com/buildingSMART/BCF-XML/tree/release_3_0/Documentation#one-to-many-mapping-between-viewpoints-and-comments

In the schema, we have this for the comment:
https://github.com/buildingSMART/BCF-XML/blob/release_3_0/Schemas/markup.xsd#L153-L167

Which means that a single comment may refer either one or zero viewpoints.

For the viewpoints, there is no longer a way to link them to a specific comment. So looks like we can drop this.

Originally raised by @pasi-paasiala in Slack.