prompt: add documentation for return value display

modules/prompt/README.md

@@ -26,6 +26,15 @@ in *zpreztorc*.
 zstyle ':prezto:module:prompt' pwd-length 'short'
 ```
 
+### Display Return Value
+
+Some prompts display the return value in the prompt. If a prompt has support,
+this can be disabled with the following snippet.
+
+```sh
+zstyle ':prezto:module:prompt' show-return-val 'no'
+```
+
 Theming
 -------
 

runcoms/zpreztorc

@@ -117,6 +117,10 @@ zstyle ':prezto:module:prompt' theme 'sorin'
 # for longer or 'full' (with '~' expansion) for even longer prompt display.
 # zstyle ':prezto:module:prompt' pwd-length 'short'
 
+# Set the prompt to display the return code along with an indicator for non-zero
+# return codes. This is not supported by all prompts.
+# zstyle ':prezto:module:prompt' show-return-val 'yes'
+
 #
 # Ruby
 #