pkg/homedir: clarify Get() docs wrt static linking
pkg/homedir: clarify Get() docs wrt static linking

This clarifies comments about static linking made in commit a8608b5b67c.

1. There are two ways to create a static binary, one is to disable
cgo, the other is to set linker flags. When cgo is disabled,
there is no need to use osusergo build tag.

2. osusergo only needs to be set when linking against glibc.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

AkihiroSuda/docker,jimjag/docker,vdemeester/docker,kojiromike/docker,gnailuy/docker,pradipd/moby,thaJeztah/docker,flavio/docker,trebonian/docker,dims/docker,jameszhan/docker,jimjag/docker,SvenDowideit/docker,estesp/docker,cpuguy83/docker,thaJeztah/docker,Ramzec/docker,jvanz/docker,jameszhan/docker,jthelin/docker,mahak/docker,konstruktoid/docker-1,pradipd/moby,bearstech/moby,jvanz/docker,Mokolea/docker,nalind/docker,thaJeztah/docker,yosifkit/docker,Mokolea/docker,cpuguy83/docker,justincormack/docker,moby/moby,mahak/docker,trebonian/docker,kojiromike/docker,estesp/docker,konstruktoid/docker-upstream,paralin/docker,Mokolea/docker,nawawi/docker,Mokolea/docker,moul/docker,estesp/docker,trebonian/docker,cpuguy83/docker,yosifkit/docker,docker/docker,ibuildthecloud/docker,ibuildthecloud/docker,ibuildthecloud/docker,dims/docker,Ramzec/docker,jimjag/docker,moul/docker,flavio/docker,konstruktoid/docker-1,konstruktoid/docker-upstream,dims/docker,nalind/docker,paralin/docker,AkihiroSuda/docker,docker/docker,yosifkit/docker,Ramzec/docker,justincormack/docker,Ramzec/docker,djs55/docker,djs55/docker,mahak/docker,yosifkit/docker,SvenDowideit/docker,trebonian/docker,ibuildthecloud/docker,nalind/docker,moby/moby,konstruktoid/docker-1,jvanz/docker,moul/docker,jameszhan/docker,bearstech/moby,flavio/docker,bearstech/moby,cpuguy83/docker,kojiromike/docker,gnailuy/docker,justincormack/docker,docker/docker,thaJeztah/docker,moby/moby,AkihiroSuda/docker,nawawi/docker,paralin/docker,paralin/docker,mahak/docker,pradipd/moby,vdemeester/docker,konstruktoid/docker-upstream,bearstech/moby,djs55/docker,ibuildthecloud/docker,nawawi/docker,konstruktoid/docker-upstream,flavio/docker,moul/docker,AkihiroSuda/docker,gnailuy/docker,jthelin/docker,nalind/docker,vdemeester/docker,moby/moby,nawawi/docker,SvenDowideit/docker,gnailuy/docker,jimjag/docker