According to http://docs.oracle.com/javase/8/docs/technotes/tools/windows/javadoc.html the "-extdirs" option "specifies the directories where extension classes reside. These are any classes that use the Java Extension mechanism. The extdirs option is part of the search path the javadoc command uses to look up source and class files. See the -classpath option for more information...".
The description is not too accurate, but it seems that it should behave as the "-classpath" option, e.g. "javadoc -extdirs \user\src com.mypackage". Actually it does not work that way (and I do not observe any implementing code).