Clarify Flash usage instructions in about:preferences

fix #4754

Auditors: @alexwykoff

Test Plan: go to about:preferences#security, scroll to the Flash switch. you should see a message linking to the Flash troubleshooting wiki.

Author: diracdeltas

app/extensions/brave/locales/en-US/preferences.properties

@@ -154,8 +154,9 @@ usePDFJS=Enable HTML5 PDF reader (requires browser restart)
 enableFlash=Enable Adobe Flash support (requires browser restart)
 widevineSection=Google Widevine Support
 enableWidevine=Enable Google Widevine support
-enableFlashSubtext=Flash support is experimental and requires Pepper Flash to be installed from
-enableFlashSubtextLinux=Flash support is experimental and requires the pepperflashplugin-nonfree package.
+enableFlashSubtext=Brave uses a special version of Pepper Flash which must be installed from
+enableFlashSubtextLinux=Flash support requires the pepperflashplugin-nonfree Linux package.
+flashTroubleshooting=Flash not working? Try the troubleshooting tips on our
 managePasswords=Manage passwords…
 sitePermissions=Saved Site Permissions
 sitePermissionsExceptions=Saved Site Exceptions

js/about/preferences.js

@@ -1578,6 +1578,13 @@ class SecurityTab extends ImmutableComponent {
                 }, true)}>{'Adobe'}</span>.</span>
               : <span data-l10n-id='enableFlashSubtextLinux' />
           }
+          <div>
+            <span className='fa fa-info-circle' id='flashInfoIcon' />
+            <span><span data-l10n-id='flashTroubleshooting' />&nbsp;
+              <span className='linkText' onClick={aboutActions.newFrame.bind(null, {
+                location: 'https://github.com/brave/browser-laptop/wiki/Flash-Support-Deprecation-Proposal#troubleshooting-flash-issues'
+              }, true)}>{'wiki'}</span>.</span>
+          </div>
         </span>
       </SettingsList>
       { !isLinux