Where g:, b:, t: and w: variables are described?

[ZyX]

I recently found that there exists `g:' (without any text after `:') variable 
which is a dictionary that holds all other global variables, same for `b:', 
`t:' 
and `w:' variables. Can anybody point me in which help topic they are described?

By the way, the following hack works:
    let g:["000"]="test"
    echo g:000

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