(PUP-3137) update doc for the user resource expiry parameter

This commit updates the documentation for the *expiry* parameter
for the 'user' resource type.

Prior to this commit, the documentation was worded incorrectly with
"does never expire" which has been updated to a more simply worded:
"If you want to ensure the user account never expires, ..."
Merge pull request #3013 from casharma/bug/PUP-3137

(PUP-3137) update doc for the user resource expiry parameter