Add API doc for files_container support

Also adds releasenote for this new feature.

Change-Id: I6da965770f2a0583277165cc6f39ae57caf43d93
Story: #1755453
Task: 22714

api-ref/source/v1/parameters.yaml

@@ -787,6 +787,12 @@ files:
   in: body
   required: false
   type: object
+files_container:
+  description: |
+    Name of the container in swift that has child templates and environment files.
+  in: body
+  required: false
+  type: string
 function_description:
   description: |
     The textual description about the usage of the template function.

api-ref/source/v1/stacks.inc

@@ -34,6 +34,7 @@ Request Parameters
    - environment: environment
    - environment_files: environment_files
    - files: files
+   - files_container: files_container
    - parameters: parameters
    - stack_name: stack_name_request
    - tags: tags
@@ -94,6 +95,8 @@ Request Parameters
    - tenant_id: tenant_id
    - disable_rollback: disable_rollback
    - environment: environment
+   - environment_files: environment_files
+   - files_container: files_container
    - files: files
    - parameters: parameters
    - stack_name: stack_name_request
@@ -368,6 +371,7 @@ Request Parameters
    - disable_rollback: disable_rollback
    - environment: environment
    - environment_files: environment_files
+   - files_container: files_container
    - existing: existing
    - files: files
    - parameters: parameters
@@ -435,6 +439,7 @@ Request Parameters
    - disable_rollback: disable_rollback
    - environment: environment
    - environment_files: environment_files
+   - files_container: files_container
    - existing: existing_patch
    - files: files
    - parameters: parameters
@@ -500,6 +505,7 @@ Request Parameters
    - disable_rollback: disable_rollback
    - environment: environment
    - environment_files: environment_files
+   - files_container: files_container
    - existing: existing
    - files: files
    - parameters: parameters
@@ -571,6 +577,7 @@ Request Parameters
    - disable_rollback: disable_rollback
    - environment: environment
    - environment_files: environment_files
+   - files_container: files_container
    - existing: existing_patch
    - files: files
    - parameters: parameters

releasenotes/notes/support-files-from-swift-f27fd2d788d67580.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - Added stack API support to provide a swift container
+    that contains the child templates and environment files.
+    All files would be fetched and used (if required), unless
+    they are superceded by files in files map.