added documentation about number_of_handles in all examples. (backport #341)

[mergify]

This is an automatic backport of pull request #341 done by Mergify.
Cherry-pick of 810afe1 has failed:

On branch mergify/bp/foxy/pr-341
Your branch is up to date with 'origin/foxy'.

You are currently cherry-picking commit 810afe1.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   rclc_examples/src/example_client_node.c
	modified:   rclc_examples/src/example_executor.c
	modified:   rclc_examples/src/example_executor_only_rcl.c
	modified:   rclc_examples/src/example_executor_trigger.c
	modified:   rclc_examples/src/example_parameter_server.c
	modified:   rclc_examples/src/example_service_node.c
	modified:   rclc_examples/src/example_short_timer_long_subscription.c
	modified:   rclc_examples/src/example_sub_context.c

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   rclc_examples/src/example_action_client.c
	deleted by us:   rclc_examples/src/example_action_server.c
	both modified:   rclc_examples/src/example_lifecycle_node.c
	both modified:   rclc_examples/src/example_pingpong.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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