This make `SPC h SPC` as default binding for accessing `helm-spacemacs`,
by either A) changing all the documentation; and B) adding a deprecation
message for `SPC f e h`.
The deprecation use a warning, because if a message is used it's hidden
directly when the helm buffer is shown, making it nearly unnoticeable
for the user.
The keybindings in octave-mode don't follow the same standards as the
others. Also use the function `spacemacs/set-leader-keys-for-major-mode`
for setting mode specific keys.
