refactor(material/sidenav): adds comments to explain reason for change

Updates previous changes to add the reasoning and reference for the
removal of the -1 for side mode sidenavs.

Author: essjay05

src/material/sidenav/drawer.ts

@@ -165,6 +165,8 @@ export class MatDrawerContent extends CdkScrollable implements AfterContentInit
     // this was also done by the animations module which some internal tests seem to depend on.
     // Simulate it by toggling the `hidden` attribute instead.
     '[style.visibility]': '(!_container && !opened) ? "hidden" : null',
+    // The sidenav container should not be focused on when used in side mode. See b/286459024 for
+    // reference. Updates tabIndex of drawer/container to default to null if in side mode.
     '[attr.tabIndex]': '(mode !== "side") ? "-1" : null',
   },
   changeDetection: ChangeDetectionStrategy.OnPush,

src/material/sidenav/sidenav.ts

@@ -47,6 +47,8 @@ export class MatSidenavContent extends MatDrawerContent {}
   templateUrl: 'drawer.html',
   host: {
     'class': 'mat-drawer mat-sidenav',
+    // The sidenav container should not be focused on when used in side mode. See b/286459024 for
+    // reference. Updates tabIndex of drawer/container to default to null if in side mode.
     '[attr.tabIndex]': '(mode !== "side") ? "-1" : null',
     // must prevent the browser from aligning text based on value
     '[attr.align]': 'null',