Explain Rietveld patching better on site.

site/dev/chrome/multi_repo_trybots.md

 Multiple repo Chromium trybots
 ==============================
 
 When a proposed Skia change will require a change in Chromium or Blink it is
 often helpful to locally create the Chromium and Blink changes and test with the
 proposed Skia change. This often happens with Skia API changes and changes
 which affect Blink layout tests. While simple to do locally, this explains how
 to do so on the Chromium trybots.
 
 The basic idea is to make your Chromium and Blink change in the usual way, but
@@ skipping 16 matching lines @@
                    '--server', 'https://codereview.chromium.org',
                    '--force',
                    '--ignore_deps',
                    '-v',
                    '-v',
                    '--no-auth',
                    '--blacklist', 'DEPS'
         ],
       },
 
+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 .
 
 
 External changes not in rietveld
 --------------------------------
 If the patch is to files where the above is not possible, then it is still
 possible to patch the files manually by adding the following to
 \<chromium>/src/DEPS in the 'hooks' array just before the 'gyp' hook.
 
@@ skipping 13 matching lines @@
 \<chromium>/src/patch/third_party/skia/include/core/SkPath.h.
 
 An example of this being used can be seen at
 https://crrev.com/1866773002/#ps20001 .
 
 
 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.