Add Pipeline help for three flags

https://www.jenkins.io/doc/pipeline/steps/copyartifact/

Add descriptions of flags

src/main/resources/hudson/plugins/copyartifact/CopyArtifact/help-fingerprintArtifacts.html

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+<div>
+  Automatically fingerprint all artifacts that are copied as part of this build step.
+</div>

src/main/resources/hudson/plugins/copyartifact/CopyArtifact/help-flatten.html

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+<div>
+  Ignore the directory structure of the artifacts in the source project and copy all matching artifacts directly into the specified target directory.
+  By default the artifacts are copied in the same directory structure as the source project.
+</div>

src/main/resources/hudson/plugins/copyartifact/CopyArtifact/help-optional.html

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+<div>
+  Allow this build to continue even if no build is found that matches the "Which build" condition.
+  By default this build step fails the build if no artifacts are copied.
+</div>