chore(cx-api): adding missing readme's (#33796)

### Issue #

Closes #33795

### Reason for this change

filling in missing readme changes from PRs #33360 and #33742

### Description of changes

Added the comment wall back in, not sure what happened there.
//////////////////////////////////////////////////////////////////////

Added readme's based on the ff descriptions 

https://github.com/aws/aws-cdk/blob/d1611f703a84faef375629dce489924ee60fab28/packages/aws-cdk-lib/cx-api/README.md?plain=1#L632-L660

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: IkeNefcy

Diff for: packages/aws-cdk-lib/cx-api/README.md

@@ -627,4 +627,34 @@ _cdk.json_
     "@aws-cdk/aws-lambda:createNewPoliciesWithAddToRolePolicy": true
   }
 }
-```
+```
+
+* `@aws-cdk/aws-s3:setUniqueReplicationRoleName`
+
+When this feature flag is enabled, a unique role name is specified only when performing cross-account replication.
+When disabled, 'CDKReplicationRole' is always specified.
+
+_cdk.json_
+
+```json
+{
+  "context": {
+    "@aws-cdk/aws-s3:setUniqueReplicationRoleName": true
+  }
+}
+```
+
+* `@aws-cdk/pipelines:reduceStageRoleTrustScope`
+
+When this feature flag is enabled, the root account principal will not be added to the trust policy of stage role.
+When this feature flag is disabled, it will keep the root account principal in the trust policy.
+
+_cdk.json_
+
+```json
+{
+  "context": {
+    "@aws-cdk/pipelines:reduceStageRoleTrustScope": true
+  }
+}
+```

Diff for: packages/aws-cdk-lib/cx-api/lib/features.ts

@@ -1389,6 +1389,8 @@ export const FLAGS: Record<string, FlagInfo> = {
     introducedIn: { v2: '2.178.0' },
     recommendedValue: true,
   },
+
+  //////////////////////////////////////////////////////////////////////
   [LAMBDA_CREATE_NEW_POLICIES_WITH_ADDTOROLEPOLICY]: {
     type: FlagType.BugFix,
     summary: 'When enabled, Lambda will create new inline policies with AddToRolePolicy instead of adding to the Default Policy Statement',
@@ -1400,6 +1402,8 @@ export const FLAGS: Record<string, FlagInfo> = {
     introducedIn: { v2: '2.180.0' },
     recommendedValue: true,
   },
+
+  //////////////////////////////////////////////////////////////////////
   [SET_UNIQUE_REPLICATION_ROLE_NAME]: {
     type: FlagType.BugFix,
     summary: 'When enabled, CDK will automatically generate a unique role name that is used for s3 object replication.',
@@ -1411,6 +1415,7 @@ export const FLAGS: Record<string, FlagInfo> = {
     introducedIn: { v2: '2.182.0' },
     recommendedValue: true,
   },
+
   //////////////////////////////////////////////////////////////////////
   [PIPELINE_REDUCE_STAGE_ROLE_TRUST_SCOPE]: {
     type: FlagType.ApiDefault,