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.
Helm seems to treat "!" specially in pattern matching, so having a ! in
the pattern string when traversing directories is problematic. This
change fixes#2737, because as far as I can tell "+" has no special
meaning in a helm pattern.
Of course, we can choose a different character, but I'm fond of "+" as
representing "more layers here".
