Gentoo libevent package installs no documentation. However, if when looking to its headers as the only remaining docs source, it's visible they look like doxygen-like or something to be built.
There are two useful options for docs:
USE=html: --enable-doxygen-doc enables doxygen documentation to be built (html by default)
USE=man: --enable-doxygen-man enables man pages generation, but requires above one flag (needs --disable-doxygen-html to disable html).
Samples (sources) would be great to be installed to docs as well with 'examples' flag.
I also checked --enable-doxygen-dot, this seems to be useless (found no dot or gv files, neither in source nor in build dirs after make command).