Add spec to shell docs

[garazdawi]

Is your feature request related to a problem? Please describe.

For markdown docs the spec is currently not displayed for shell docs:

 foldl(Fun, Acc0, List)

  Calls Fun(Elem, AccIn) on successive elements A of List,
  starting with AccIn == Acc0. Fun/2 must return a new

Describe the solution you'd like

The spec should be show as it does in 26.2

  -spec foldl(Fun, Acc0, List) -> Acc1
                 when
                     Fun :: fun((Elem :: T, AccIn) -> AccOut),
                     Acc0 :: term(),
                     Acc1 :: term(),
                     AccIn :: term(),
                     AccOut :: term(),
                     List :: [T],
                     T :: term().

  Calls Fun(Elem, AccIn) on successive elements A of List, starting with AccIn == Acc0. Fun/2 must return