Security Center: docstring changes via synth.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/securitycenter/artman_securitycenter_v1beta1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:229601142daed005bf56fbaa21a02cf7aa0b261b57677d38306f8013e536de7a
Status: Image is up to date for googleapis/artman:latest
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/securitycenter-v1beta1.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/source_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/organization_settings_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/finding_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/asset_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/securitycenter_service_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/security_marks_pb2.py.
synthtool > Cleaned up 0 temporary directories.