Undocumented behaviour - parameter expansion ${par-word}

[Roman Rakus]

Hi,
Petr, adding to cc: list, found behaviour not documented, neither in man 
page nor bash ref manual:
${par-word} will do expansion of par, and if the par is unset it is 
substituted by word. It is different from ${par:-word}, where word is 
used when par is unset or null.

Is it undocumented and deprecated, like $[]? Or just undocumented?

RR