Improve syntax for inline documentation and allow for multi-line usage

libexec/rbenv-global

@@ -1,4 +1,17 @@
 #!/usr/bin/env bash
+#
+# Summary: Set or show the global Ruby version
+#
+# Usage: rbenv global <version>
+#
+# Sets the global Ruby version. You can override the global version at
+# any time by setting a directory-specific version with `rbenv local'
+# or by setting the `RBENV_VERSION' environment variable.
+#
+# <version> should be a string matching a Ruby version known to rbenv.
+# The special version string `system' will use your default system Ruby.
+# Run `rbenv versions' for a list of available Ruby versions.
+
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x