[prompt] Rename helper function promptpwd to prompt-pwd for consistency

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`.

modules/prompt/functions/prompt-pwd

@@ -1,7 +1,11 @@
-# prompt setup function common to many prompts
-# moved to external function to reduce code redundancy
+#
+# Prompt setup function commonly used by prompt themes.
+#
+# Authors:
+#   Sorin Ionescu <sorin.ionescu@gmail.com>
+#
 
-# function promptpwd {
+# function prompt-pwd {
 
 setopt localoptions extendedglob
 
@@ -19,6 +23,8 @@ else
   ret_directory="${${${${(@j:/:M)${(@s:/:)current_pwd}##.#?}:h}%/}//\%/%%}/${${current_pwd:t}//\%/%%}"
 fi
 
+unset current_pwd
+
 print "$ret_directory"
 
 # }

modules/prompt/functions/prompt_damoekri_setup

@@ -16,7 +16,7 @@ prompt_damoekri_precmd() {
   unsetopt XTRACE KSH_ARRAYS
 
   # Format PWD.
-  _prompt_damoekri_pwd=$(promptpwd)
+  _prompt_damoekri_pwd=$(prompt-pwd)
 
   # Get Git repository information.
   if (( $+functions[git-info] )); then

modules/prompt/functions/prompt_paradox_setup

@@ -84,7 +84,7 @@ function prompt_paradox_precmd {
   unsetopt XTRACE KSH_ARRAYS
 
   # Format PWD.
-  _prompt_paradox_pwd=$(promptpwd)
+  _prompt_paradox_pwd=$(prompt-pwd)
 
   # Get Git repository information.
   if (( $+functions[git-info] )); then

modules/prompt/functions/prompt_sorin_setup

@@ -82,7 +82,7 @@ function prompt_sorin_precmd {
   unsetopt XTRACE KSH_ARRAYS
 
   # Format PWD.
-  _prompt_sorin_pwd=$(promptpwd)
+  _prompt_sorin_pwd=$(prompt-pwd)
 
   # Define prompts.
   RPROMPT='${editor_info[overwrite]}%(?:: %F{1}⏎%f)${VIM:+" %B%F{6}V%f%b"}'