[Feat] Document option --help-text

[BadAssassin]

--help-text is an undocumented option for modules that outputs to stdout (stderr is default with --help). This option needs to be documented.

[neteler]

https://github.com/OSGeo/grass/blob/main/lib/gis/parser.c#L466

has been introduced in 8ffa3de.

Background

I could trace it back here (found by checking communication in "grass-dev" for the date of the above commit):

https://lists.osgeo.org/pipermail/grass-dev/2010-February/049096.html

which leads to the original ticket:

"module help information in machine-readable format"
https://trac.osgeo.org/grass/ticket/953

[BadAssassin]

FYI, this issue was created as response to #1131.