refactor(dialog): use type overload for open method

* Reworks the `MatDialog.open` method to use overloads, rather than the single signature that has `componentOrTemplateRef: ComponentType<T> | TemplateRef<T>`. This makes the separation clearer and looks nicer in autocompletion.
* Adds docs for the `MatBottomSheet.open` method.

Author: crisbeto

src/material/bottom-sheet/bottom-sheet.ts

@@ -61,8 +61,21 @@ export class MatBottomSheet implements OnDestroy {
       @Optional() @Inject(MAT_BOTTOM_SHEET_DEFAULT_OPTIONS)
           private _defaultOptions?: MatBottomSheetConfig) {}
 
+  /**
+   * Opens a bottom sheet containing the given component.
+   * @param component Type of the component to load into the bottom sheet.
+   * @param config Extra configuration options.
+   * @returns Reference to the newly-opened bottom sheet.
+   */
   open<T, D = any, R = any>(component: ComponentType<T>,
                    config?: MatBottomSheetConfig<D>): MatBottomSheetRef<T, R>;
+
+  /**
+   * Opens a bottom sheet containing the given template.
+   * @param template TemplateRef to instantiate as the bottom sheet content.
+   * @param config Extra configuration options.
+   * @returns Reference to the newly-opened bottom sheet.
+   */
   open<T, D = any, R = any>(template: TemplateRef<T>,
                    config?: MatBottomSheetConfig<D>): MatBottomSheetRef<T, R>;
 

src/material/dialog/dialog.ts

@@ -120,11 +120,22 @@ export class MatDialog implements OnDestroy {
 
   /**
    * Opens a modal dialog containing the given component.
-   * @param componentOrTemplateRef Type of the component to load into the dialog,
-   *     or a TemplateRef to instantiate as the dialog content.
+   * @param component Type of the component to load into the dialog.
    * @param config Extra configuration options.
    * @returns Reference to the newly-opened dialog.
    */
+  open<T, D = any, R = any>(component: ComponentType<T>,
+                            config?: MatDialogConfig<D>): MatDialogRef<T, R>;
+
+  /**
+   * Opens a modal dialog containing the given template.
+   * @param template TemplateRef to instantiate as the dialog content.
+   * @param config Extra configuration options.
+   * @returns Reference to the newly-opened dialog.
+   */
+  open<T, D = any, R = any>(template: TemplateRef<T>,
+                            config?: MatDialogConfig<D>): MatDialogRef<T, R>;
+
   open<T, D = any, R = any>(componentOrTemplateRef: ComponentType<T> | TemplateRef<T>,
           config?: MatDialogConfig<D>): MatDialogRef<T, R> {