This was a change in zsh 5.4.1 where prompt_sp is now reset on calls to
prompt, unlike previous behavior where it was set by default. This
restores the previous default behavior.
Refs #1423, but is not a complete fix because we need to wait for
external prompts to update as well.
Changes:
* In prezto, function names are hyphenated ('-') by convention,
rename `promptpwd` to `prompt-pwd` accordingly.
* Unset local variable `current_pwd` proactively.
* Tweak additional documentation for `prompt-pwd`.
