cmd/ld: consistently document flags that expect values
cmd/ld: consistently document flags that expect values

LGTM=minux, rsc
R=golang-codereviews, minux, rsc
CC=golang-codereviews
https://codereview.appspot.com/113970044

Committer: Shenghou Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

bryanxu/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc