v.clean: Change threshold description to clarify map units

[neteler]

This PR de-confuses the current description of threshold from the misleading

  threshold   Threshold in map units, one value for each tool

to

  threshold   One value for each tool; for threshold units, see each tool

The point is that users need to look it up in the manual page. Esp. in long-lat locations, map units for area based tools are to be given in sqm, see https://grass.osgeo.org/grass-stable/manuals/v.clean.html

[metzm]

The manual would need adjustment: either explain threshold in general: map units with the exception of tool=rmarea where units are always squaremeter, or add an explanation of the threshold to each tool in the manual.

[echoix]

The manual would need adjustment: either explain threshold in general: map units with the exception of tool=rmarea where units are always squaremeter, or add an explanation of the threshold to each tool in the manual.

Personally, I'd prefer going the long route, and have an explanation in each tool in the manual, or at least to refer to a common page at each tool. Since when using a tool, you'd be using the docs of the tool you are using, not the other ones or global docs (imagine the documentation section in the user interface tab when using the tool)

[wenzeslaus]

The label is improved without creating any negative side effects. The documentation of the v.clean tool already mentions units for at least some of its tools. Let's merge this as is.

[landam]

@neteler Please consider merging this PR.