auth/aws: Fix outdated help texts (#5253)

builtin/credential/aws/backend.go

@@ -286,13 +286,22 @@ func getAnyRegionForAwsPartition(partitionId string) *endpoints.Region {
 }
 
 const backendHelp = `
-aws-ec2 auth method takes in PKCS#7 signature of an AWS EC2 instance and a client
-created nonce to authenticates the EC2 instance with Vault.
+The aws auth method uses either AWS IAM credentials or AWS-signed EC2 metadata
+to authenticate clients, which are IAM principals or EC2 instances.
 
 Authentication is backed by a preconfigured role in the backend. The role
 represents the authorization of resources by containing Vault's policies.
 Role can be created using 'role/<role>' endpoint.
 
+Authentication of IAM principals, either IAM users or roles, is done using a
+specifically signed AWS API request using clients' AWS IAM credentials. IAM
+principals can then be assigned to roles within Vault. This is known as the
+"iam" auth method.
+
+Authentication of EC2 instances is done using either a signed PKCS#7 document
+or a detached RSA signature of an AWS EC2 instance's identity document along
+with a client-created nonce. This is known as the "ec2" auth method.
+
 If there is need to further restrict the capabilities of the role on the instance
 that is using the role, 'role_tag' option can be enabled on the role, and a tag
 can be generated using 'role/<role>/tag' endpoint. This tag represents the

builtin/credential/aws/path_role.go

@@ -891,12 +891,12 @@ Create a role and associate policies to it.
 
 const pathRoleDesc = `
 A precondition for login is that a role should be created in the backend.
-The login endpoint takes in the role name against which the instance
-should be validated. After authenticating the instance, the authorization
-for the instance to access Vault's resources is determined by the policies
-that are associated to the role though this endpoint.
+The login endpoint takes in the role name against which the client
+should be validated. After authenticating the client, the authorization
+to access Vault's resources is determined by the policies that are
+associated to the role though this endpoint.
 
-When the instances require only a subset of policies on the role, then
+When an EC2 instance requires only a subset of policies on the role, then
 'role_tag' option on the role can be enabled to create a role tag via the
 endpoint 'role/<role>/tag'. This tag then needs to be applied on the
 instance before it attempts a login. The policies on the tag should be a