Explain Rietveld patching better on site.

site/dev/chrome/multi_repo_trybots.md

Index: site/dev/chrome/multi_repo_trybots.md
diff --git a/site/dev/chrome/multi_repo_trybots.md b/site/dev/chrome/multi_repo_trybots.md
index 9f3cfaa8f0a06c049c9d0a89cb78c8555bcf4d46..16502ef5d3feca95b6480e8a4e96d46931df944c 100644
--- a/site/dev/chrome/multi_repo_trybots.md
+++ b/site/dev/chrome/multi_repo_trybots.md
@@ -34,6 +34,14 @@ and the patch is already in Rietveld, then add the following to
         ],
       },
 
+Modify the 'issue' and 'patchset' to the appropriate values.
+If this is for a project other than Skia, update the 'root_dir' and 'server'.
+Note that this can be used multiple times to apply multiple issues.
+
+To find the patchset number in Rietveld use the URL of the '[raw]' (old UI) or
+'Raw Patch' (new UI) link on the desired patch. The last segment of this URL
+has the form 'issue\<issue>_\<patchset>.diff'.
+
 An example of this being used can be seen at
 https://crrev.com/1877673002/#ps120001 .
 
@@ -67,4 +75,4 @@ Try the patch
 -------------
 After committing these \<chromium>/src/DEPS and \<chromium>/src/patch/ changes
 locally, 'git cl upload' can be used in the usual way. Be sure to add
-'COMMIT=false' to the issue description to avoid accidentally checking it in.
+'COMMIT=false' to the issue description to avoid accidentally checking it in.