First of all the documentation is correct, but it's hard to find out the correct steps to follow, so I'll give here a few quick steps valid in case of JBoss 4.x and Java 5 or Java 6.
- delete server/default/deploy/jboss-aop-jdk50.deployer and replace it with jboss-40-install/jboss-aop-jdk50.deployer
- move into the new folder and delete common-softvaluehashmap.jar
(already available) and move javassist.jar to ../../lib/javassist.jar (to replace the jboss providen library)
- edit server/default/deploy/jboss-aop-jdk50.deployer/META-INF/jboss-service.xml and set the EnableLoadtimeWeaving parameter to true
- edit the run.bat or run.sh script and add the java option -javaagent:$JBOSS_HOME/bin/pluggable-instrumentor.jar