I’m pretty new to HA, but have good experience in general.
I’ve been searching docs, community, etc… and have not found where the attributes of various devices are documented.
Question: What am I missing? Where is that info hiding?
Or perhaps that’s an HPDL? (Hard Part, Do Later) 
My specific instance is “Interactive”… it goes on and off for my phone, but I have no idea what it means.
Open the Android companion app, Settings, companion app, sensors. Scroll to see all of them - interactive is there among them. Click on it. There’s a short “descriptive” line saying: Interactive it is on when the device is in an interactive state. Wouldn’t have guessed that one. 
But hey: top right there’s an icon: if you click it there’s more. Just learned myself I like this sensor more than the one I was using to check if my kids are only pretending to sleep.
The documentation is actually pretty good
Mine simply says “Interactive: True” right now. No explanation for any of the sensors.
But YES – HOORAY! The help has a table of sensors with brief description, and down below a longer explanation.
For my particular item (“Interactive State”)… yeah, I’m curious how well it knows whether I’m interacting or not. Certainly, screen-on is not a useful predictor for me. But I bet screen touches might be.
Anyway… yep, that document is where the info I need is found. AND for the future, the following Google search brings it right up: home assistant “interactive sensor”
AND, at the top-right of that page is a search tool that works nicely. Entering “interactive” brings me directly to what I want.
The interesting bit: one must recognize in advance that this sensor is tied to the Companion App, even though the information is integrated into the whole HA ecosphere.
Maybe I forgot to say to click on it. Then you get a screen for that sensor alone. The icon takes you to the docs. I thought this would help you find it easier for other sensors too. Plus I really liked making fun of the non descriptive description, that simply uses the same word again. That happens a lot.
So far I was looking at device locked to see if some one was using the phone, but that is only updated once per 15 mins. Interactive is immediate.