Clarify what the labels of a source block do. (#11236)

Co-authored-by: Sylvia Moss <moss@hashicorp.com>

website/content/docs/templates/hcl_templates/blocks/source.mdx

@@ -12,29 +12,38 @@ The top-level `source` block defines reusable builder configuration blocks:
 
 `@include 'from-1.5/sources/example-block.mdx'`
 
+The first label — `happycloud` here — is the builder type. The second label is
+the unique name or identifier you want to give to the source. There can be only one
+`source.happycloud.foo` top-level source block; but it can be used more than
+once. Builders are usually found in plugins, except for the `file` and the
+`null` builders that will remain in Packer core.
+
 You can start builders by refering to those source blocks from a [`build`
-block](/docs/templates/hcl_templates/blocks/build), for example :
+block](/docs/templates/hcl_templates/blocks/build) :
 
 ```hcl
 build {
   sources = [
     # Here Packer will use a default ami_name when saving the image.
-    "source.amazon-ebs.example",
-    "source.amazon-ebs.foo",
+    "source.happycloud.example",
+    "source.happycloud.foo",
   ]
 }
 ```
 
-The build-level [`source` block](/docs/templates/hcl_templates/blocks/build/source) allows to
-set specific source fields.
+The build-level [`source`
+block](/docs/templates/hcl_templates/blocks/build/source) allows to set specific
+source fields. Each field must be defined only once and it is currently not
+allowed to override a value.
 
 ```hcl
 build {
-  source "source.amazon-ebs.example" {
-    # Here Packer will use the provided ami_name instead of defaulting it.
+  source "source.happycloud.example" {
+    # Here Packer will use the provided image_name instead of defaulting it.
     # Note that fields cannot be overwritten, in other words, you cannot
-    # set the 'ami_name' field in the top-level source block.
-    ami_name = "specific"
+    # set the 'image_name' field in the top-level source block and here at the
+    # same time
+    image_name = "build_specific_field"
   }
 }
 ```

website/content/partials/from-1.5/sources/example-block.mdx

@@ -1,6 +1,6 @@
 ```hcl
 # sources.pkr.hcl
-source "amazon-ebs" "example-1" {
+source "happycloud" "foo" {
     // ...
 }
 ```