config.md: various cleanup/consistency fixes
config.md: various cleanup/consistency fixes

- standardise on "filesystem" instead of the previous mix of "file
  system" and "filesystem"
- change "Machine-specific" to the more accurate "Platform-specific"
- move towards zeroth article for all field specifications
- grammar tweaks

zenlinTechnofreak/specs,hqhq/specs,ujuc/specs,jhjeong-kr/specs,vbatts/specs,philips/specs,cloudfoundry-incubator/specs,navidshaikh/specs,kensipe/specs,wking/opencontainer-runtime-spec,talwai/specs,wangkirin/specs,ChengTiesheng/specs,mheon/specs,hqhq/runtime-spec,vishh/specs,sladage/specs,wking/opencontainer-specs,LK4D4/specs,rootfs/specs,dustymabe/specs,runcom/specs,opencontainers/runtime-spec,HardikDR/specs,opencontainers/specs,jonboulle/specs,gao-feng/specs,marcosnils/specs,haijunTan/specs,laijs/specs