[doc] In the helps, give an example of using "Source" in a rule

[bbryson]

I'm having a hard time figuring out when one would use anything from the Source side (either affix glosses or features) in a rule, since it all comes over in transfer. It would be helpful to have an example in the docs. I can imagine needing to know the source glosses, but we have access to them in the transferred string, right?

I also don't know when one would use the Source features at all.

[rmlockwood]

You might want to do something in a rule if the source lemma is something in particular or if the source category is a certain thing.