gnash installs documentation into /usr/share/doc/${PN} but it should go to /usr/share/doc/${PF}. At the moment there are two doc dirs (/usr/share/doc/gnash, /usr/share/doc/gnash-0.8.6) as dodoc installs into /usr/share/doc/${PF}.
Passing --docdir=/usr/share/doc/${PF} to configure did not help. Possibly the build system needs to be patched.
Fixed in CVS.