2 minor fixes, including for the bug reported by macaj@dcs.fmph.uniba.sk

[hulpke]

As this was a reported bug on short notice.

I have not been able to reduce the reported error examples to anything plausible for test files.

2nd attempt, now based off stable.

Also added the requested documentation fix for #613 that is already in master.

[olexandr-konovalov]

Thanks @hulpke - there is no promise that this will go into 4.8.2, but then we may include the fix into 4.8.3.

My only concern is that this PR changes the level of many assertions, and this may have side-effect on tests that depend on randomness. Wasn't our intention to follow the newly suggested guidelines for assertion levels (#564) in the master branch, and not in stable-4.8 ?

[hulpke]

I have tried to change the changes to leave out the assertion changes. Somehow this deleted @fingolfin 's comment -- sorry!.

As far as I'm concerned it does not need to go into 4.8.2, its just an option.

[fingolfin]

@hulpke You didn't do anything wrong! My comments were attached to the commit, and since you replaced the commit, they are not visible anymore -- but that's intended behavior. After all, you fully addressed my request with your change (thank you).

There is no concern with this anymore, so I am merging it now.