macOS Catalina + Java 8
$ sw_vers
ProductName: Mac OS X
ProductVersion: 10.15.3
BuildVersion: 19D76
$ java -version
openjdk version "1.8.0_242"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_242-b08)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.242-b08, mixed mode)
Download and unpack the Minecraft Forge source code. This time, download version 1.15 series.
$ wget https://github.com/MinecraftForge/MinecraftForge/archive/1.15.x.zip
$ unzip 1.15.x.zip
Generate an API reference HTML page with the javadoc command.
$ javadoc -d docs -sourcepath MinecraftForge-1.15.x/src/main/java -subpackages net.minecraftforge
You can see that HTML files etc. are generated in the directory specified by the -d option.
$ find docs | head
docs
docs/constant-values.html
docs/overview-tree.html
docs/net
docs/net/minecraftforge
docs/net/minecraftforge/registries
docs/net/minecraftforge/registries/IRegistryDelegate.html
docs/net/minecraftforge/registries/IForgeRegistry.CreateCallback.html
docs/net/minecraftforge/registries/ForgeRegistry.Snapshot.html
docs/net/minecraftforge/registries/ObjectHolderRef.html