In handbook section "4.d. Writing Init Scripts", please add a link to the
OpenRC migration document. Alternatively, add everything below to the 4.d
block, and a link BACK from the OpenRC migration document to the portion of the
handbook.

The following goes into openrc-migration.xml:
Add a new toplevel L1 block, after the 'Finishing Up' L2 block:

=============

<h1>Converting init.d scripts</h1>
<note>
  This section is intended primarily for developers, but is also relevant to
  anybody with custom init.d scripts.
</note>
<h2>Detecting OpenRC</h2>
<p>
  If your init.d script needs to detect being run under OpenRC specifically to
  perform different actions, you are strongly encourage to use the following as
  the means of detecting OpenRC:
</p>
<code description="OpenRC detection test, transitional">
  [[ -f /lib/librc.so -o -f /etc/init.d/sysfs -o -f /libexec/rc/version ]]
</code>

<code description="Example usage">
depend() {
  if [[ -f /lib/librc.so -o -f /etc/init.d/sysfs -o -f /libexec/rc/version ]]; then
    need sysfs
  fi
}
start() {
  echo foobar >/sys/...
}
</code>

<p>
  After $DATE [1], the official detection test will be:
</p>
<code description="OpenRC detection test, final">
  [[ -f /libexec/rc/version ]]
</code>
<h2>Detecting the OpenRC version</h2>
<p>
  If you need to know the exact version of OpenRC, you should RDEPEND on the
  minimum version, and then read the file as follows:
</p>
<code description="OpenRC version query">
  read version </libexec/rc/version
</code>

=============
I've just put $DATE in here since we don't know when yet.