Add a warning about needing to restart the entire login session

E.g. for a GUI session, ~/.profile is executed by the GUI login "shell" at its startup
so one needs to fully log out and log back in.
Before that, the change would only be seen by shells explicitly started as login shells.

libexec/pyenv-init

@@ -114,6 +114,9 @@ function help_() {
       ;;
     esac
     echo
+    echo '# Make sure to restart your entire logon session'
+    echo '# for changes to ~/.profile to take effect.'
+    echo
   } >&2
 }