plugin_services.md: clarify light service scope (IJSDK-853)
plugin_services.md: clarify light service scope (IJSDK-853)

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