Hi
I think that particular element in the documentation is referring to wikis and blogs only unfortunately. Ironically, the document library used to work this way in earlier versions but was changed to allow easier entry of multi-word tags.
Probably the quickest way to add tags is to use the full Tags dialog that you get access to in the Edit Properties screen/popup. From here you can type the name of each tag and click the button with the check mark next to it. If the tag doesn't exist it will be created and added, if it does exist it will just be added. If you leave your mouse over the check mark button so you can click immediately after entering a tag, you can add a good number of tags pretty quickly.
Not ideal though - the alternative is to create a modified version of the insitu editor used for tags entry and get it to split the tags based on something like a semi-colon;
Regards
Steven