Adds comment for php73 runtime beta (GoogleCloudPlatform#891)

Author: bshaffer

appengine/php72/auth/app.yaml

@@ -1,3 +1,4 @@
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 # Defaults to "serve index.php" and "serve public/index.php". Can be used to

appengine/php72/cloudsql/app-postgres.yaml

@@ -2,6 +2,7 @@
 # See the default app.yaml for running Cloud SQL with MySQL.
 
 # [START gae_cloudsql_postgres_env]
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 env_variables:

appengine/php72/cloudsql/app.yaml

@@ -2,6 +2,7 @@
 # See app-postgres.yaml for running Cloud SQL with PostgreSQL.
 
 # [START gae_cloudsql_mysql_env]
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 env_variables:

appengine/php72/errorreporting/app.yaml

@@ -1,3 +1,4 @@
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 # Defaults to "serve index.php" and "serve public/index.php". Can be used to

appengine/php72/front-controller/app.yaml

@@ -1,3 +1,4 @@
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 # Defaults to "serve public/index.php" and "serve index.php". Can be used to

appengine/php72/getting-started/app.yaml

@@ -1,6 +1,7 @@
 # See https://cloud.google.com/appengine/docs/standard/php/config/appref for a
 # complete list of `app.yaml` directives.
 
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 env_variables:

appengine/php72/grpc/app.yaml

@@ -1,3 +1,4 @@
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 handlers:

appengine/php72/helloworld/app.yaml

@@ -1,3 +1,4 @@
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 # Defaults to "serve index.php" and "serve public/index.php". Can be used to

appengine/php72/laravel-framework/app-dbsessions.yaml

@@ -1,3 +1,4 @@
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 env_variables:

appengine/php72/laravel-framework/app.yaml

@@ -1,3 +1,4 @@
+# Use the PHP 7.3 runtime (BETA) by replacing "php72" below with "php73"
 runtime: php72
 
 env_variables: