docs: update comments in the Docker build script

PiperOrigin-RevId: 523243599

.kokoro/docker_update.sh

@@ -4,6 +4,18 @@
 # PHP and Ruby runtimes binaries to use in GAPIC generators for these
 # languages.
 #
+# Historically, PHP and Ruby keep their compiled runtimes in the generators'
+# repositories:
+# https://github.com/googleapis/gapic-generator-php/tree/main/rules_php_gapic/resources
+# https://github.com/googleapis/gapic-generator-ruby/tree/main/rules_ruby_gapic/prebuilt
+# They are needed to prevent rebuilding of PHP and Ruby runtime (each taking
+# several minutes) every time the build is executed from a clean workspace.
+#
+# Sometimes these binaries need to be updated: they are dynamically linked,
+# and may stop working if some dependency changes in an incompatible way.
+# Use this script to update both the googleapis Docker image, and those
+# binaries.
+#
 # Run from any local working directory where you have write access.
 #
 # Usage: