doc: more detailed description of -tags option

[yaojingguo]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.7.1 linux/amd64

The description -tags option is:

-tags 'tag list'
    a list of build tags to consider satisfied during the build.
    For more information about build tags, see the description of
    build constraints in the documentation for the go/build package.

tag list needs to be separated by spaces. And https://golang.org/pkg/go/build/ talks about using spaces or commas to separate tags. I think that it will be clearer if a list of build tags is changed to a list of space-separated build tags.

[quentinmit]

I'm going to close this as a dupe of #17148; resolving that will include updating the documentation to clarify what is accepted.