Add some $i18n{} information to the webui_explainer.md.

Change-Id: Ie15668cb56f165060f85d6fa11735b6823ad1793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877872
Auto-Submit: calamity <calamity@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710202}

docs/webui_explainer.md

@@ -157,6 +157,12 @@ Visiting `chrome://donuts` should show in something like:
 
 Delicious success.
 
+By default $i18n{} escapes strings for HTML. $i18nRaw{} can be used for
+translations that embed HTML, and $i18nPolymer{} can be used for Polymer
+bindings. See
+[this comment](https://bugs.chromium.org/p/chromium/issues/detail?id=1010815#c1)
+for more information.
+
 ## C++ classes
 
 ### WebUI