add docs for fileset and its example

I thought about that also, Alvin, when I started using the literal includes. You can tag, or go back text instead of line numbers. My thinking was that this is not automatic; if you change the code in such a way to change the line count, then you need to check the resulting html. You can also add something to the file to comment that it is being included and any edits must be careful. 
The most fool-proof system I have devised is one used for files in the other repos: I cache locally copies of files I am using, and if they change, the build system warns you that they have changed and you need to check the changes. We could something like this here.