Software documentation is my second long-term career, and my third or fourth overall (depending on how you count).
One thing that I found useful when I was starting out was mapping my first-career skills to software skills explicitly in my resume. I had been an academic, which meant that I could emphasize my research, writing, and teaching/training experience. I had also done a little web development work, and I'd played around on Unix/VAX/the Internet-before-the-web a bit.
Everyone's background is different, but two fundamental skills for software docs are research and writing/communication, so you might try finding areas in a previous career where you'd used these skills, and build from what you already know and can do.
Hope it helps, and best of luck!