docs: add recipe upgrade docs

cli/recipe/recipe.go

@@ -151,8 +151,20 @@ var recipeCreateCommand = &cli.Command{
 }
 
 var recipeUpgradeCommand = &cli.Command{
-	Name:      "upgrade",
-	Usage:     "Upgrade recipe image tags",
+	Name:  "upgrade",
+	Usage: "Upgrade recipe image tags",
+	Description: `
+This command reads and attempts to parse all image tags within the given
+<recipe> configuration and prompt with more recent tags to upgrade to. It will
+update the relevant compose file tags on the local file system.
+
+Some image tags cannot be parsed because they do not follow some sort of
+semver-like convention. In this case, all possible tags will be listed and it
+is up to the end-user to decide.
+
+This is step 1 of upgrading a recipe. Step 2 is running "abra recipe sync
+<recipe>".
+`,
 	ArgsUsage: "<recipe>",
 	Action: func(c *cli.Context) error {
 		recipe := c.Args().First()