bug: source-sh broken, improve documentation

I'm trying to use source-sh to get some variables from an Alpine Linux APKBUILD into the environment of the Ion shell.

The APKBUILD looks like this:

# Contributor: dai9ah <dai9ah@protonmail.com>
# Maintainer: dai9ah <dai9ah@protonmail.com>
pkgname=2bwm
pkgver=0.3
pkgrel=0
pkgdesc="Fast floating window manager"
url="https://github.com/venam/2bwm"
arch="all"
license="ISC"
makedepends="libxcb-dev xcb-util-keysyms-dev xcb-util-wm-dev xcb-util-xrm-dev"
options="!check" # no test suite
subpackages="$pkgname-doc"
source="$pkgname-$pkgver.tar.gz::https://github.com/venam/2bwm/archive/v$pkgver.tar.gz"

prepare() {
	default_prepare
	sed -i Makefile -e "/CFLAGS/{s/+=-Os /+=/}"
}

build() {
	make
}

package() {
	make PREFIX=/usr DESTDIR="$pkgdir" install
}

sha512sums="088a97e5245287890c72e2b0685f7348a4cc0fd49582893b7ce7a081f80a4d7454a3c0eadf4609589314351ded02fd8b75548019b782e797350ad5db5c939f92  2bwm-0.3.tar.gz"

After sourcing it with source-sh i expect the variables in the root of the document to be accessible from Ion.