[css-align] use of start/end edge needs qualification

In [1], the phrases "start edge" and "end edge" are used without qualifying
which axis applies.

Also, I notice the terms "start side" and "end side" also appear under the
definitions of self-start and self-end. The term "side" is not defined in
this spec nor is a definition referenced. For that matter, the term "edge"
is similarly undefined. At the minimum, "edge" and "side" need to be
carefully distinguished, with example illustrations if possible.

[1] http://dev.w3.org/csswg/css-align/#ltcontent-position