Copyedit pass over args library description; fixed some example code
Copyedit pass over args library description; fixed some example code

R=mcampione@google.com, rnystrom@google.com

Review URL: https://codereview.chromium.org//23603002

git-svn-id: 965eb99b257a0350f91a5ab8607b3dea57271703@27388 260f80e4-7a28-3924-810f-c04153c831b5

iansf/args,dart-lang/args,ronlobo/args,seaneagan/args