Add documentation for Images API v2 show-image-member call

Image sharing was added to Images v2.1, but the show image
member call was not documented.  This patch adds the missing
documentation.

New and improved using the present tense!

Change-Id: I9924a91c30e150e7e9a5a7b62f099336b56e62d3
Closes-Bug: #1282381
Merge "Add documentation for Images API v2 show-image-member call"