Enhace keymap docs (incl. shell & toggle_visual)

[DLFW]

• Better description for spawn and shell, now mentioning %s and with “%s-examples”.
• Describe toggle_visual and escape.
• Add example for deselecting all files
• Few random, tiny enhancements

[DLFW]

This MR was created and merged before as #217. That MR however got reverted somehow. I don't know what happened there, maybe @krivahtoo knows more, looks for me like he reverted that commit.

Assuming that the revert did not happen intentionally , I send this branch once more for merge.

[krivahtoo]

@DLFW I didn't. I accidentaly clicked the 'revert' button, i don't think it did any thing apart from Github mentioning on the PR.

[DLFW]

@krivahtoo Alright, no worries. :smile: I just realized that my commit somehow disappeared afterwards. I expected that - whatever the cause was - it was not intentional.