[fuchsia] Update CIPD command line example to use "latest" package ref.

The "latest" package ref can be used by clients to discover the
most recently built package instance. Given that instance, the client
can use "cipd describe" to get the package version, which can be used
to select the instances of related packages that were built and pushed
at the same repo revision.


Bug: 869197
Change-Id: I09de4669a93e07f2546f0dfea088f733870ee267
Reviewed-on: https://chromium-review.googlesource.com/1173053Reviewed-by: James Robinson <jamesr@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582703}

webrunner/cipd/fidl.yaml

@@ -7,8 +7,14 @@
 # To create a CIPD package, run the following command from the build output
 # directory.
 #
-# cipd create --pkg-def ../../webrunner/cipd/fidl.yaml \
-#             -tag version:$(cat fuchsia_artifacts/build_id.txt)
+# $ cipd create --pkg-def ../../webrunner/cipd/fidl.yaml \
+#               -ref latest \
+#               -tag version:$(cat fuchsia_artifacts/build_id.txt)
+#
+# The most recent package can be discovered by searching for the "latest" ref:
+#
+# $ cipd describe chromium/fuchsia/fidl -version latest
+
 package: chromium/fuchsia/fidl
 description: FIDL definitions for the "chromium" service.
 root: ../fidl

webrunner/cipd/webrunner.yaml

@@ -14,10 +14,16 @@
 # To create a CIPD package, run the following command from the build output
 # directory.
 #
-# cipd create --pkg-def ../../webrunner/cipd/webrunner.yaml \
-#             -pkg-var targetarch:$TARGET_ARCH \
-#             -pkg-var outdir:`pwd` \
-#             -tag version:$(cat fuchsia_artifacts/build_id.txt)
+# $ cipd create --pkg-def ../../webrunner/cipd/webrunner.yaml \
+#               -pkg-var targetarch:$TARGET_ARCH \
+#               -pkg-var outdir:`pwd` \
+#               -ref latest \
+#               -tag version:$(cat fuchsia_artifacts/build_id.txt)
+#
+# The most recent package can be discovered by searching for the "latest" ref:
+#
+# $ cipd describe chromium/fuchsia/webrunner-$TARGET_ARCH -version latest
+
 package: chromium/fuchsia/webrunner-${targetarch}
 description: Prebuilt Chrome and Web Runner binaries for Fuchsia.
 root: ${outdir}/fuchsia_artifacts