[editor's note: Dota 2 displays the inventory hotkeys without mentioning the "ALT +" part. I guess it assumes you know how you've set up your own hotkeys.]
When I was playing DotA 1 I used autohotkey to bind my slots to Q and 4. When I started to play HoN their standard was ALT + Q, ALT + W and so on and I got used to that, so when I went back to Dota 2 I kept the same hotkeys I used in HoN.