foundation for a help system where each command holds its own docs

Docs are comprised from "Usage", "Summary" and "Help" sections, where
"Help" can span multiple commented lines. If it is missing, "Summary" is
shown in its place.

References #204, references #206

libexec/rbenv-whence

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
+# Usage: rbenv whence [--path] COMMAND
+# Summary: List all Ruby versions with the given command installed
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x
 
@@ -27,7 +29,7 @@ whence() {
 
 RBENV_COMMAND="$1"
 if [ -z "$RBENV_COMMAND" ]; then
-  echo "usage: rbenv whence [--path] COMMAND" >&2
+  rbenv-help whence | head -1 >&2
   exit 1
 fi