docs(material/dialog): clarify that default options replace rather than merge

Add a note to the dialog documentation explaining that providing
MAT_DIALOG_DEFAULT_OPTIONS (or DEFAULT_DIALOG_CONFIG for CDK) replaces
the entire built-in config rather than merging with it. Include the
workaround of spreading the defaults for partial overrides.

Also update the code examples to use bootstrapApplication and
"app config" prose for consistency.

Closes #20631

Author: yogeshwaran-c

src/cdk/dialog/dialog.md

@@ -117,16 +117,28 @@ class MyDialogContainer extends CdkDialogContainer {}
 
 ### Specifying global configuration defaults
 Default dialog options can be specified by providing an instance of `DialogConfig` for
-`DEFAULT_DIALOG_CONFIG` in your application's root module.
+`DEFAULT_DIALOG_CONFIG` in your app config.
 
 ```ts
-@NgModule({
+bootstrapApplication(MyApp, {
   providers: [
     {provide: DEFAULT_DIALOG_CONFIG, useValue: {hasBackdrop: false}}
   ]
-})
+});
 ```
 
+> **Note:** The value provided for `DEFAULT_DIALOG_CONFIG` **replaces** the built-in defaults
+> entirely rather than merging with them. For example, providing `{disableClose: true}` means that
+> all other defaults (such as `hasBackdrop`) will be `undefined`. If you only want to override
+> specific properties, spread the defaults first:
+>
+> ```ts
+> {provide: DEFAULT_DIALOG_CONFIG, useValue: {...new DialogConfig(), disableClose: true}}
+> ```
+>
+> When you call `dialog.open()` with a config, that config is merged on top of these defaults, so
+> per-dialog options always take precedence.
+
 ### Sharing data with the Dialog component.
 You can use the `data` option to pass information to the dialog component.
 

src/material/dialog/dialog.md

@@ -41,16 +41,28 @@ export class YourDialog {
 
 ### Specifying global configuration defaults
 Default dialog options can be specified by providing an instance of `MatDialogConfig` for
-MAT_DIALOG_DEFAULT_OPTIONS in your application's root module.
+MAT_DIALOG_DEFAULT_OPTIONS in your app config.
 
 ```ts
-@NgModule({
+bootstrapApplication(MyApp, {
   providers: [
     {provide: MAT_DIALOG_DEFAULT_OPTIONS, useValue: {hasBackdrop: false}}
   ]
-})
+});
 ```
 
+> **Note:** The value provided for `MAT_DIALOG_DEFAULT_OPTIONS` **replaces** the built-in defaults
+> entirely rather than merging with them. For example, providing `{disableClose: true}` means that
+> all other defaults (such as `hasBackdrop`) will be `undefined`. If you only want to override
+> specific properties, spread the defaults first:
+>
+> ```ts
+> {provide: MAT_DIALOG_DEFAULT_OPTIONS, useValue: {...new MatDialogConfig(), disableClose: true}}
+> ```
+>
+> When you call `dialog.open()` with a config, that config is merged on top of these defaults, so
+> per-dialog options always take precedence.
+
 ### Sharing data with the Dialog component.
 If you want to share data with your dialog, you can use the `data`
 option to pass information to the dialog component.