docs(material/dialog): add documentation for MAT_DIALOG_DEFAULT_OPTIONS

[Davitigs]

Reproduction

Steps to reproduce:

1. provide MAT_DIALOG_DEFAULT_OPTIONS in the module
2. provide useValue. (ex. disableClose: true - see the following code)
   {
   provide: MAT_DIALOG_DEFAULT_OPTIONS,
   useValue: {
   disableClose: true,
   },
   },

Expected Behavior

What behavior were you expecting to see?

Configs should be set along with the default values for the other options. Such behaviour can be seen when provided in the component with dialog.open() method

Actual Behavior

What behavior did you actually see?

Given configs overwrite the Configs value, so that default options are also overwritten.

Environment

• Angular:
• CDK/Material:
• Browser(s):
• Operating System (e.g. Windows, macOS, Ubuntu):

[mmalerba]

This seems to be working, here's a stackblitz showing it: https://stackblitz.com/edit/angular-dxw9c8?file=src/main.ts

[Davitigs]

This seems to be working, here's a stackblitz showing it: https://stackblitz.com/edit/angular-dxw9c8?file=src/main.ts

Please try to click on "Open Dialog" button when your popup is already opened:

1. There is no backdrop
2. You can open the same popup over and over again without closing the previous one.
   However we both know that by default the backdrop is set to true and in this case there is no backdrop.

[mmalerba]

There is no automatic merging of your override with the default options. If you want to keep the rest of the default options just merge them when providing the value:

{ 
    ...new MatDialogConfig(),
    disableClose: true,
}

See: https://stackblitz.com/edit/angular-dxw9c8-2sxvwj?file=src%2Fmain.ts

[Davitigs]

why then such automatic merging is happening when the configs are provided in the .open() method?

[mmalerba]

When you call open with a config it merges the given config with the default config. In your case you're overriding the default config, so it merges that with your custom default: {disableClose: true}

[Davitigs]

I understand that, however in the official documentation there is no info provided for the case when the custom config is provided in the module, you can figure that out only when investigating the source code, I think would be better to somehow include that aspect, it is not intuitive.