/usr/share/doc/libffi6/html/ is in libffi-dev 3.2.1-6.
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/usr/share/doc/libffi6/html/Closure-Example.html
/usr/share/doc/libffi6/html/Complex-Type-Example.html
/usr/share/doc/libffi6/html/Complex.html
/usr/share/doc/libffi6/html/Index.html
/usr/share/doc/libffi6/html/Introduction.html
/usr/share/doc/libffi6/html/Missing-Features.html
/usr/share/doc/libffi6/html/Multiple-ABIs.html
/usr/share/doc/libffi6/html/Primitive-Types.html
/usr/share/doc/libffi6/html/Simple-Example.html
/usr/share/doc/libffi6/html/Structures.html
/usr/share/doc/libffi6/html/The-Basics.html
/usr/share/doc/libffi6/html/The-Closure-API.html
/usr/share/doc/libffi6/html/Type-Example.html
/usr/share/doc/libffi6/html/Types.html
/usr/share/doc/libffi6/html/Using-libffi.html
/usr/share/doc/libffi6/html/index.html