Add the following paragraph between Feed Content and Protected Entries?
Tag View URLs: View entries with a specific tag by adding ?tag=tagname to the end of the feed URL, replacing tagname with actual tag name. Separate the tags by commas to view two or more tags at once: ?tag=tag1,tag2.
I think there are too many combinations to list them all for single tag vs. multiple tags, personal vs. comm, RSS vs. Atom. This follows a similar format as http://www.livejournal.com/support/faqbrowse.bml?faqid=226 (tags) and http://www.livejournal.com/support/faqbrowse.bml?faqid=175 (appearance of comment pages). auth=digest is the only parameter commonly used there, so I left out the ? vs. & note.
Also, we could consider putting the feed view info in the tags FAQ and/or linking to 149 from 226.