Update keyword table and accompanying text

Also remove note about deferred loading issue and
improve page descriptions.

Staged at:

https://book-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html#keywords

Compare to:

https://www.dartlang.org/docs/dart-up-and-running/ch02.html#keywords

BUG= https://github.com/dart-lang/www.dartlang.org/issues/1047, https://github.com/dart-lang/www.dartlang.org/issues/1095, https://github.com/dart-lang/www.dartlang.org/issues/1136
R=shaza@google.com

Review URL: https://chromiumcodereview.appspot.com/723633002