plugin_extension_points.md: more complete sample, ExtensionPointName usage
plugin_extension_points.md: more complete sample, ExtensionPointName usage

JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs