postinst is in postgresql-doc-9.5 9.5.2-1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27  | #!/bin/sh
set -e
VERSION=${DPKG_MAINTSCRIPT_PACKAGE##*-}
# arguments: version master package [package]
__link_manpages() {
    MANS=$(unset GREP_OPTIONS; dpkg -L $3 $4 $5 2>/dev/null|egrep '/man/.*\.[1-9](\.gz)?$' | grep -v "$2") || true
    [ -n "$MANS" ] || return 0
    SLAVES=$(for i in $MANS; do TARGET=$(echo $i | sed "s/postgresql\/$1\///"); echo -n " --slave $TARGET $(basename $i) $i"; done)
    section=$(echo "$2" | sed -e 's/.*\.\(.*\)\..*/man\1/')
    update-alternatives --install /usr/share/man/$section/$2 \
        $2 /usr/share/postgresql/$1/man/$section/$2 \
        $(echo "$1" | tr -cd 0-9) $SLAVES
}
if [ "$1" = configure ]; then
    if [ -f /usr/share/postgresql-common/maintscripts-functions ]; then
        . /usr/share/postgresql-common/maintscripts-functions
        configure_doc_version $VERSION "$2"
    else
        __link_manpages $VERSION SPI_connect.3.gz "postgresql-doc-$VERSION"
    fi
fi
 |