Fix documentation of configuring extraDirectories for Gradle plugin (#3682)
Fix documentation of configuring extraDirectories for Gradle plugin (#3682)

jib.extraDirectories = file('build/extra-directory') is one of the older syntax shown here, that is later deprecated in #1671 and removed in #2108.
GoogleContainerTools/jib,GoogleContainerTools/jib,GoogleContainerTools/jib,GoogleContainerTools/jib