doc: copy the `Include Guards` section from the -cpp repo (googleapis/google-cloud-cpp-common#217)
doc: copy the `Include Guards` section from the -cpp repo (googleapis/google-cloud-cpp-common#217)

* doc: copy the `Include Guards` section from the -cpp repo

All of our repos follow this convention so it belongs in common.

* fix stray trailing space

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp