Fix along with negative distance was undefined

[Leffe108]

This changes so that if distance is negative, then it measures along the line in reverse. Before, it would use bearing and return a point before the start.

Also document what happens if abs(distance) is larger than the length of the line.

Resolves #2567, resolves #2148

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[magnusburton]

Much needed change, thanks! It might be breaking for some who've come to expect this pattern however.

[smallsaucepan]

Thanks for taking a look at this @Leffe108. Have made a request above if you could expand the tests to cover some more of the functionality that would be fantastic.