You're getting this bug because the package in summary installs its documentation (or at least part of it) outside the usual /usr/share/doc/${PF} directory. First, please keep in mind that this bug might not be noticeable for -r0 ebuilds, but it might be for -r1 and later, since if the ebuild has same name and version of the package, for -r0 it might correspond properly. To fix this, if the package uses autotools, recent version (autoconf 2.61+) have two ./configure switches: --docdir and%2
This package's Makefile include /usr/lib64/postgresql-9.1/lib64/pgxs/src/makefiles/pgxs.mk and it install pgmecab's doc into postgres' doc/contrib directroy. It also install its sql file into /usr/share/postgresql-9.1/contrib/pgmecab.sql. Shoud we drop doc anyway?
Package dropped.