Add explanation on duration argument help and README regarding the se…

…ssion duration defaults

README.rst

@@ -132,7 +132,8 @@ Usage
       -R REGION, --region REGION
                             AWS region endpoint ($AWS_DEFAULT_REGION)
       -d DURATION, --duration DURATION
-                            Credential duration ($DURATION)
+                            Credential duration (defaults to value of $DURATION, then
+                            falls back to 43200)
       -p PROFILE, --profile PROFILE
                             AWS profile (defaults to value of $AWS_PROFILE, then
                             falls back to 'sts')
@@ -150,7 +151,7 @@ Usage
       -V, --version         show program's version number and exit
 
 
-**Note** that if you want longer than the default 3600 seconds (1 hour)
+**Note** If you want a longer session than the AWS default 3600 seconds (1 hour)
 duration, you must also modify the IAM Role to permit this. See
 `the AWS documentation <https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_manage_modify.html>`__
 for more information.

aws_google_auth/__init__.py

@@ -27,7 +27,7 @@ def parse_args(args):
     parser.add_argument('-I', '--idp-id', help='Google SSO IDP identifier ($GOOGLE_IDP_ID)')
     parser.add_argument('-S', '--sp-id', help='Google SSO SP identifier ($GOOGLE_SP_ID)')
     parser.add_argument('-R', '--region', help='AWS region endpoint ($AWS_DEFAULT_REGION)')
-    parser.add_argument('-d', '--duration', type=int, help='Credential duration ($DURATION)')
+    parser.add_argument('-d', '--duration', type=int, help='Credential duration in seconds (defaults to value of $DURATION, then falls back to 43200)')
     parser.add_argument('-p', '--profile', help='AWS profile (defaults to value of $AWS_PROFILE, then falls back to \'sts\')')
     parser.add_argument('-D', '--disable-u2f', action='store_true', help='Disable U2F functionality.')
     parser.add_argument('-q', '--quiet', action='store_true', help='Quiet output')