docs: explain SnapshotCredentials (#20431)

fixes #20388 

I'm interested in why `DatabaseClusterFromSnapshot` generates an `admin` username unlike the other snapshot constructs, I'm unfamiliar with why it's be okay to generate a username for that but not an instance or serverless cluster

----

### All Submissions:

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### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: peterwoodworth

packages/@aws-cdk/aws-rds/README.md

@@ -185,7 +185,7 @@ const rule = instance.onEvent('InstanceEvent', { target: new targets.LambdaFunct
 
 ## Login credentials
 
-By default, database instances and clusters will have `admin` user with an auto-generated password.
+By default, database instances and clusters (with the exception of `DatabaseInstanceFromSnapshot` and `ServerlessClusterFromSnapshot`) will have `admin` user with an auto-generated password.
 An alternative username (and password) may be specified for the admin user instead of the default.
 
 The following examples use a `DatabaseInstance`, but the same usage is applicable to `DatabaseCluster`.
@@ -232,6 +232,27 @@ new rds.DatabaseInstance(this, 'InstanceWithCustomizedSecret', {
 });
 ```
 
+### Snapshot credentials
+
+As noted above, Databases created with `DatabaseInstanceFromSnapshot` or `ServerlessClusterFromSnapshot` will not create user and auto-generated password by default because it's not possible to change the master username for a snapshot. Instead, they will use the existing username and password from the snapshot. You can still generate a new password - to generate a secret similarly to the other constructs, pass in credentials with `fromGeneratedSecret()` or `fromGeneratedPassword()`.
+
+```ts
+declare const vpc: ec2.Vpc;
+const engine = rds.DatabaseInstanceEngine.postgres({ version: rds.PostgresEngineVersion.VER_12_3 });
+const myKey = new kms.Key(this, 'MyKey');
+
+new rds.DatabaseInstanceFromSnapshot(this, 'InstanceFromSnapshotWithCustomizedSecret', {
+  engine,
+  vpc,
+  snapshotIdentifier: 'mySnapshot',
+  credentials: rds.SnapshotCredentials.fromGeneratedSecret('username', {
+    encryptionKey: myKey,
+    excludeCharacters: '!&*^#@()',
+    replicaRegions: [{ region: 'eu-west-1' }, { region: 'eu-west-2' }],
+  }),
+});
+```
+
 ## Connecting
 
 To control who can access the cluster or instance, use the `.connections` attribute. RDS databases have