Slightly expand BL Touch explanation of Pin configuration

[do3cc]

Stating why the pullup is needed might help people who found wrong examples in the internet for probe configuration.

[JamesH1978]

Thank you for submitting a PR, please be aware that you need to sign off to accept the developer certificate of origin, please see point 3 in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md#what-to-expect-in-a-review

Thanks
James

[Sineos]

TBH, I'm not sure if this not rather increases the confusion.
Most users do not know what the pull-up actually does and even less so what an open-drain is. For those who know both terms and what they do, the explanation is not needed.

[do3cc]

Added Signed off and follow the preferred format.

[do3cc]

Maybe I am an outlier. I was stuck by this for two nights, being new to klipper and having used the Probe successfully with Marlin.
I just see now that even the sample config from Bigtreetech is wrong about it.
During my extensive googling, I got the impression, must people do not understand the consequences of Open Drain and Pullups. I do not dabble with microcontrollers regularly enough, so I only started to realize what was happening after I got and working.
The Failure mode being working somewhat doesn't help either. Therefor I thought it is a good addition.

But I trust your judgement whether it reduces help requests by BLTouch users or adds confusion.

Independent of that, I will create a PR for the Bigtreetech sample config.

[do3cc]

There is already an ignored Pull Request and at least 7 open issues regarding that PIN and at least mentioning Klipper. 19 Bugs when also looking for Marlin. Meh, I didn't know how BTT treats the support of their hardware.
I will create a new pull request for the example config of the SKR Mini Board with a section for BLTouch which is outcommented. My problem seems to be SKR Mini specific.

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