It causes confusion that when javadoc runs out of memory, it says to
use -J-Xmx, but the docs makefile itself uses -J-xm. Here is the error
message:

javadoc: java.lang.OutOfMemoryError: Please increase memory.
      For example, on the Sun Classic or HotSpot VMs, add the option -J-Xmx
      such as -J-Xmx32m.
      1 error

The X is actually optional for backward compatibility reasons.
(When calling java, the -J must be dropped)

It would be clearer to add X throughout the docs makefile.