Add additional workaround information for software update issue

@phnakarin I sourced this information from
https://cfengine.zendesk.com/entries/25642616-Client-Upgrade-from-3-5-0-3-5-1-3-5-2-to-3-5-3.
Can you please validate it?

I included diff output to more clearly highlight the changes required.
@deb-cfengine, @tzz, @zzamboni any thoughts on showing diff output or not?
Pygments has DiffLexer, but I don't know if we have it available to us via the
documentation generation system or not. Here an example
http://pygments.org/demo/170619/