updated explanatory text and alt text for data table example in HTML 5.1

Hi all,

I have resolved a bug[1] about missing alt text on image that shows how the
scope attribute effects what cells a header cell applies to.

In the process I expanded the explanatory text that accompanies the code
example [2], as I think the original [3] did not adequately convey the
information to all users.

I would appreciate it if people reviewed the changed explanatory text.

[1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=20916

[2]
http://www.w3.org/html/wg/drafts/html/master/tabular-data.html#the-th-element(changed
text starts: The header cells in row 1 ('ID', 'Measurement',
'Average' and 'Maximum') each apply only to the cells in their column.)

[3]http://www.w3.org/TR/html51/tabular-data.html#the-th-element (original
text starts: "The headers in the first row all apply directly down to the
rows in their column...)

with regards

--
SteveF
HTML 5.1 <http://www.w3.org/html/wg/drafts/html/master/>
<http://www.paciellogroup.com/resources/wat-ie-about.html>