Clarify --trailing-comma a bit

Refs. #2655

README.md

@@ -549,12 +549,13 @@ Default | CLI Override | API Override
 `false` |  `--single-quote` | `singleQuote: <bool>`
 
 ### Trailing Commas
-Print trailing commas wherever possible.
+Print trailing commas wherever possible when multi-line. (A single-line array,
+for example, never gets trailing commas.)
 
 Valid options:
  * `"none"` - No trailing commas.
  * `"es5"` - Trailing commas where valid in ES5 (objects, arrays, etc.)
- * `"all"` - Trailing commas wherever possible (function arguments). This requires node 8 or a [transform](https://babeljs.io/docs/plugins/syntax-trailing-function-commas/).
+ * `"all"` - Trailing commas wherever possible (including function arguments). This requires node 8 or a [transform](https://babeljs.io/docs/plugins/syntax-trailing-function-commas/).
 
 Default | CLI Override | API Override
 --------|--------------|-------------

bin/prettier.js

@@ -305,7 +305,7 @@ if (
       "  --no-bracket-spacing     Do not print spaces between brackets.\n" +
       "  --jsx-bracket-same-line  Put > on the last line instead of at a new line.\n" +
       "  --trailing-comma <none|es5|all>\n" +
-      "                           Print trailing commas wherever possible. Defaults to none.\n" +
+      "                           Print trailing commas wherever possible when multi-line. Defaults to none.\n" +
       "  --parser <flow|babylon|typescript|postcss|json|graphql>\n" +
       "                           Specify which parse to use. Defaults to babylon.\n" +
       "  --cursor-offset <int>    Print (to stderr) where a cursor at the given position would move to after formatting.\n" +