[CP-beta]Remove docs_deploy_beta, fix docs_publish, add comments. (#168824)

This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/blob/main/docs/releases/Flutter-Cherrypick-Process.md#automatically-creates-a-cherry-pick-request)
Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request.

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

Closes https://github.com/flutter/flutter/issues/168709.

### Changelog Description:
Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples

N/A (CI Infra Change)

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)

Not user facing - this fixes API doc generation for release candidate branches, which was accidentally broken in https://github.com/flutter/flutter/pull/162557.

### Workaround:
Is there a workaround for this issue?

N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

Merge a commit to `flutter-3.32-candidate.0` and observe the post-submits.

.ci.yaml

@@ -582,19 +582,25 @@ targets:
       # tests are sharded.
       test_timeout_secs: "3600" # Allows 60 minutes (up from 30 default)
 
+  # Despite the name, this really means "generate api_docs.zip", and "upload zip
+  # to GCS", and conditionally on the "master" channel will also deploy the docs
+  # to Firebase (https://main-api.flutter.dev/).
+  #
+  # See "Linux docs_deploy_stable" for how the docs are deployed to stable API.
   - name: Linux docs_publish
     recipe: flutter/docs
     presubmit: false
     backfill: false
+    # This means "allow this to be scheduled by the release/release_builder"
+    # recipe. Normally we'd use "schedule: true", but that *also* means "do not
+    # run this in normal presubmit/postsubmit", and we do want it to run in
+    # postsubmit for the "master" channel. Sorry.
+    #
+    # See https://github.com/flutter/flutter/issues/168709 for details.
+    schedule_during_release_override: true
     timeout: 60
     dimensions:
       os: "Linux"
-    enabled_branches:
-      # Produces docs for main-api.flutter.dev
-      # stable and beta are managed by the targets:
-      #   - Linux docs_deploy_beta
-      #   - Linux docs_deploy_stable
-      - master
     properties:
       cores: "32"
       dependencies: >-
@@ -6915,30 +6921,10 @@ targets:
     drone_dimensions:
       - os=Windows
 
-
-  - name: Linux docs_deploy_beta
-    recipe: flutter/docs
-    scheduler: release
-    bringup: true
-    enabled_branches:
-      - beta
-    presubmit: false
-    timeout: 60
-    properties:
-      cores: "32"
-      dependencies: >-
-        [
-          {"dependency": "dashing", "version": "0.4.0"},
-          {"dependency": "firebase", "version": "v11.0.1"}
-        ]
-      tags: >
-        ["framework", "hostonly", "linux"]
-      validation: docs_deploy
-      validation_name: Docs_deploy
-      firebase_project: master-docs-flutter-dev
-    drone_dimensions:
-      - os=Linux
-
+  # This step runs on the release channel "stable", after the same commit SHA
+  # has been run and built by Linux flutter_release_builder as part of a release
+  # candidate branch (i.e. /flutter-\d+\.\d+-candidate\.\d+/) in the previous
+  # target, "Linux docs_publish".
   - name: Linux docs_deploy_stable
     recipe: flutter/docs
     scheduler: release