[PATCH v4 1/2] doc: add devargs documentation

[Thomas Monjalon]

> Currently, the devargs syntax documentation is missing from the
> programmer's guide, and is only documented implicitly through the devargs
> API headers. Add a detailed description of the two supported devargs
> syntaxes.
> 
> Acked-by: John McNamara <john.mcnamara at intel.com>

Acked-by: Thomas Monjalon <thomas at monjalon.net>

Series applied with a bit of formatting, thanks.