Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally

Packages describing “doc” as local USE flag

Package“doc” Flag Description
app-crypt/mit-krb5Creates and installs the API and implementation documentation. This is only useful if you want to develop software which depends on kerberos
app-emulation/open-vm-toolsGenerate API documentation
app-i18n/pyzyGenerate API documentation
app-misc/muttprintBuilds documentation
app-portage/eixCreate description of the eix cache file additionally in html format
dev-libs/boostInstall the full API documentation documentation. This takes over 200MB of extra disk space.
dev-libs/libp11Generate and install API documentation for the package.
dev-libs/libunibreakInstall html API documentation.
dev-lisp/clxBuilds documentation
dev-lisp/cmuclInstalls package documentation
dev-python/icalendarGenerate examples and other extra documentation
dev-util/perfBuild documentation and man pages. With this USE flag disabled, the --help parameter for perf and its sub-tools will not be available. This is optional because it depends on a few documentation handling tools that are not always welcome on user systems.
games-engines/solarusInstall doxygen developer documentation.
games-rpg/dreamwebHigh-resolution scans of the written material in the original UK game box
mail-client/alotInstall user manual and API documentation
media-libs/libpulseBuild the doxygen-described API documentation.
media-sound/easyeffectsInstall packages needed to display built-in user documentation
media-sound/pulseeffectsInstall packages needed to display built-in user documentation
net-analyzer/cactiinstall html documentation
net-analyzer/muninBuild and install a local copy of the HTML documentation for the whole software. This requires dev-python/sphinx to process the documentation sources.
net-firewall/nftablesCreate man pages for the package (requires app-text/asciidoc)
net-libs/c-clientInstall RFCs related to IMAP
net-mail/notmuchInstall API documentation
sys-devel/llvmBuild and install the HTML documentation and regenerate the man pages
sys-libs/ldbBuilds documentation
x11-misc/alockBuild and install manpage with app-text/asciidoc.
x11-misc/picomBuild documentation and man pages (requires app-text/asciidoc)
x11-wm/i3Build and install the HTML documentation and regenerate the man pages
x11-wm/stumpwmInclude documentation

All packages providing a “doc” USE flag (3001)