website: explain how to upgrade providers

Previously the -upgrade option was covered only on the "terraform init" usage page. It seems also worth mentioning in the main docs on provider versioning, since we're already explaining here other mechanics of the versioning/constraints system.

website/docs/configuration/providers.html.md

@@ -110,6 +110,12 @@ This special argument applies to _all_ providers.
 view the specified version constraints for all providers used in the
 current configuration.
 
+When `terraform init` is re-run with providers already installed, it will
+use an already-installed provider that meets the constraints in preference
+to downloading a new version. To upgrade to the latest acceptable version
+of each provider, run `terraform init -upgrade`. This command also upgrades
+to the latest versions of all Terraform modules.
+
 ## Multiple Provider Instances
 
 You can define multiple instances of the same provider in order to support