docs: Added link to AFW Updates within N overview.

The first paragraph under the "Android for Work" section on the
"Android N for Developers" page now includes a link to the "Android
for Work Updates" page. Also reworded the sentence containing this
link for a smoother transition into the hyperlink text.

Bug: 27785758
Change-Id: I5e3510c32a108106c4142e74f9fe597b9c25ae69