Fix documentation of QuadraticCoreAirgap

[christiankral]

In #3307 a leakage model was added in parallel to the air gap (and coil) model:

Therefore the included image does not match the model as it does not include the leakage effect:

In addition to that the table and the text do not match the model any more:

I made an attempt to fix some parts of the documentation. The following issues are still missing:

• @AHaumer Please update the sketch which shows only one dashed line going straight through the airgap
• The included table does currently not show the leakage path as it has not "real" area of cross section and length; @AHaumer I wonder if can somehow overcome this problem or just leave the table as it is

[christiankral]

@beutlich Thanks for fixing these issues
By the way: I like Markdown a lot more than HTML ...

[beutlich]

By the way: I like Markdown a lot more than HTML ...

Luckily we have HTML Tidy now running through Travis CI.

[AHaumer]

Thanks for fixing that ...

[dietmarw]

@AHaumer There are still the things missing that @christiankral mentioned above. Can you provide the updated sketches and table?

[AHaumer]

@dietmarw
Thanks to @christiankral - he has updated the table already!
I tried to update the sketch accordingly.