[Feat] Document pipeline option in r.proj and v.proj with a working example

[torsti]

Is your feature request related to a problem? Please describe.

Trying to use custom proj pipelines in r.proj or v.proj results in the following error messages, seemingly independent of what is passed in the pipeline option.

WARNING: A custom pipeline requires input and output projection info
ERROR: Unable to initialize coordinate transformation

The warning is raised by GPJ_init_transform(), the error by the v.proj or r.proj module.

The reprojections work if nothing is passed in pipeline, when I tested reprojections between various demo datasets.

The errors are not informative as to what should be changed in the option and the lack of documentation and examples makes it hard to start from a working example for pipeline.

Describe the solution you'd like

The use of pipeline should be documented with a minimum working example in the Examples section of the documentation pages of v.proj and r.proj.

Describe alternatives you've considered

Alternatively there is a bug in r.proj and v.proj which makes the option unusable. This would again be easier to debug given an example that is expected to work. Given the code context where the warning message is raised in GPJ_init_transform() the info_out parameter is not set, but should be?

Additional context

Tested with GRASS GIS 7.8.7 on Windows and GRASS GIS 8.2.0 on Debian.