Concat arguments when enabling haddock

[jasagredo]

Arguments were unioned with <> which is left-biased, therefore -haddock swallowed all other args as indeed described in #10782.

QA

The example in #10782 now correctly will fail even if documentation is enabled.

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[jasagredo]

I think the bug was introduced by 80f0a65, which makes the expression modified in this PR to be non-empty if documentation was enabled. I however don't know if I'm introducing some subtle bug by not ignoring the default args anymore but concatenating them with the user supplied args.