AW docs: update WebView beta steps for R

No change to logic, only docs. This changes the "Android 10" section to
be "Android 10 and later" since Trichrome still applies in R (and will
continue to be the format for the foreseeable future).

Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ia3fc77f32a778be7845ef3afba1215d5d5d5fe75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508317
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822642}

android_webview/docs/prerelease.md

@@ -45,7 +45,7 @@ version of WebView around.
 
 Steps depend on your version of Android:
 
-### Android 10 (Q)
+### Android 10 and later (Q, R, etc.)
 
 Pre-release channels must be downloaded and installed as separate apps, but only
 one must be chosen to provide the system's WebView implementation at any given
@@ -53,7 +53,8 @@ time.
 
 *** note
 WebView pre-release channels are independent of Chrome in Android 10 and later.
-Chrome can no longer be used as a WebView implementation in Android 10.
+Chrome can no longer be used as a WebView implementation in Android 10 and
+later.
 ***
 
 1. Download a pre-release channel of WebView from the play store, available here: