FOP (and many other Apache projects) use Apache Gump to do a test build several times each day. One of the useful byproducts of this process is that javadocs can be created and made available.
First, determine which line of development code you wish to see. If you don't know, you probably want the "Maintenance Branch", which is the line that currently produces FOP releases. See Development Lines for more details.
See Source Download for instructions on obtaining the source code. Then see Building FOP for instructions on running the build process. The Ant build target that you will use to generate the API documentation is "javadocs", and the results will be stored in xml-fop/build/javadocs.