Nice summary of items. Personally I usually set parameters to NULL and then check for NULL to display the "you need parameter x" included" message.
I'm wary of external documentation. People get too busy and this is difficult to keep up with, so I don't recommend it. Instead, I prefer to keep any notes, docs, etc. in the proc. Still searching for a good way to provide this to new people, but I think it needs to be kept with the code.
Steve Jones
http://www.sqlservercentral.com/columnists/sjones
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