A few tweaks to the help text

libexec/rbenv-help

@@ -1,13 +1,18 @@
 #!/usr/bin/env bash -e
 
 print_set_version() {
-  echo "<version> should be a string matching the installed Ruby name known by rbenv.
-
-For your install, this is currently one of:
-
-$(rbenv-versions --bare)
-
-The special version string 'system' will use your default system Ruby."
+  echo "<version> should be a string matching a Ruby version known by rbenv."
+
+  local versions="$(rbenv-versions --bare)"
+  if [ -z "$versions" ]; then
+    echo "There are currently no Ruby versions installed for rbenv."
+  else
+    echo "The currently installed Ruby versions are:"
+    echo "$versions" | sed 's/^/  /'
+  fi
+
+  echo
+  echo "The special version string 'system' will use your default system Ruby."
 }
 
 case "$1" in
@@ -15,27 +20,33 @@ case "$1" in
 
 Some useful rbenv commands are:
    commands      List all commands
-   rehash        Rehash rbenv shims, use after installing binaries
-   set-default   Set global default Ruby
-   set-local     Set local directory default Ruby
-   version       Show Ruby version being used
-   versions      List Ruby versions known by rbenv
+   rehash        Rehash rbenv shims (run this after installing binaries)
+   set-default   Set the default Ruby version
+   set-local     Set a local directory-specific Ruby version
+   version       Show the current Ruby version
+   versions      List all Ruby versions known by rbenv
 
 See 'rbenv help <command>' for more information on a specific command.
 For a quick guide to rbenv, see: https://gist.github.com/1120938"
 ;;
 set-default) echo "usage: rbenv set-default <version>
 
-Sets the global default Ruby. 
+Sets the default Ruby version. You can override the default at any time
+by setting a directory-specific version with \`rbenv set-default' or by
+setting the RBENV_VERSION environment variable.
 
 $(print_set_version)"
 ;;
 set-local) echo "usage: rbenv set-local <version>
 
-Sets the local directory default Ruby, by writing the version to a file
-named '.rbenv-version'. rbenv will search for this file up the directory
-tree from the current working directory each time its shims are executed,
-so this default will affect subdirectories.
+Sets the local directory-specific Ruby version by writing the version
+name to a file named '.rbenv-version'.
+
+When you run a Ruby command, rbenv will look for an '.rbenv-version'
+file in the current directory and each parent directory. If no such
+file is found in the tree, rbenv will use the default Ruby version
+specified with \`rbenv set-default', or the version specified in the
+RBENV_VERSION environment variable.
 
 $(print_set_version)"
 ;;