add a section to the manual discussing the command= variable
add a section to the manual discussing the command= variable

This includes a mention of using cmd /c on Windows.
This would have helped on issue #1070 for example.

AoD314/ninja,juntalis/ninja,sxlin/dist_ninja,mgaunard/ninja,ndsol/subninja,sxlin/dist_ninja,ndsol/subninja,ndsol/subninja,moroten/ninja,moroten/ninja,nicolasdespres/ninja,bradking/ninja,mgaunard/ninja,bradking/ninja,iwadon/ninja,lizh06/ninja,mydongistiny/ninja,mgaunard/ninja,nafest/ninja,fuchsia-mirror/third_party-ninja,iwadon/ninja,sxlin/dist_ninja,sgraham/ninja,ndsol/subninja,vvvrrooomm/ninja,nicolasdespres/ninja,mohamed/ninja,nafest/ninja,ninja-build/ninja,sgraham/ninja,sxlin/dist_ninja,nico/ninja,nafest/ninja,nico/ninja,nafest/ninja,iwadon/ninja,moroten/ninja,Qix-/ninja,Qix-/ninja,nico/ninja,sgraham/ninja,Maratyszcza/ninja-pypi,mydongistiny/ninja,atetubou/ninja,mydongistiny/ninja,mohamed/ninja,fuchsia-mirror/third_party-ninja,fuchsia-mirror/third_party-ninja,juntalis/ninja,iwadon/ninja,AoD314/ninja,maruel/ninja,nicolasdespres/ninja,Maratyszcza/ninja-pypi,juntalis/ninja,lizh06/ninja,bradking/ninja,vvvrrooomm/ninja,nicolasdespres/ninja,ninja-build/ninja,lizh06/ninja,martine/ninja,mgaunard/ninja,maruel/ninja,vvvrrooomm/ninja,martine/ninja,sxlin/dist_ninja,moroten/ninja,sgraham/ninja,Maratyszcza/ninja-pypi,atetubou/ninja,ninja-build/ninja,Qix-/ninja,mohamed/ninja,maruel/ninja,sxlin/dist_ninja,Qix-/ninja,ninja-build/ninja,bradking/ninja,AoD314/ninja,AoD314/ninja,martine/ninja,vvvrrooomm/ninja,nico/ninja,mohamed/ninja,fuchsia-mirror/third_party-ninja,maruel/ninja,atetubou/ninja,atetubou/ninja,sxlin/dist_ninja,mydongistiny/ninja,juntalis/ninja,martine/ninja,Maratyszcza/ninja-pypi,lizh06/ninja