cmd/oci-image-tool/man: Document multiple --ref for 'validate'

[wking]

Catch up with 6ca74be (cmd/oci-image-tool: support multiple --ref, 2016-09-06, #265).

While I'm touching it, reformat this entry to one line per sentance per the README's Markdown style section.

[wking]

On Tue, Sep 06, 2016 at 10:07:49PM -0700, W. Trevor King wrote:

While I'm touching it, reformat this entry to one line per sentance
per the README's Markdown style section.

Although it appears go-md2man does not actually mark up these
definition lists:

$ go-md2man -in cmd/oci-image-tool/man/oci-image-tool-validate.1.md -out /dev/stdout
…
.PP
\fB--ref\fP NAME
The reference to validate (should point to a manifest).
Can be specified multiple times to validate multiple references.
\fB\fCNAME\fR must be present in the \fB\fCrefs\fR subdirectory of the image.
Defaults to \fB\fCv1.0\fR&.
Only applicable if type is image or imageLayout.
…

I'm not really sure what the plan is here. I'd be in favor of
switching to AsciiDoc or other man generator with a bit more polish
1.

[runcom]

LGTM (IANAM)

[jonboulle]

lgtm

[philips]

LGTM