Print trailing comma for TS type parameters but not type arguments (#10353)

docs/options.md

@@ -110,13 +110,13 @@ Use single quotes instead of double quotes in JSX.
 
 _Default value changed from `none` to `es5` in v2.0.0_
 
-Print trailing commas wherever possible when multi-line. (A single-line array, for example, never gets trailing commas.)
+Print trailing commas wherever possible in multi-line comma-separated syntactic structures. (A single-line array, for example, never gets trailing commas.)
 
 Valid options:
 
-- `"es5"` - Trailing commas where valid in ES5 (objects, arrays, etc.)
+- `"es5"` - Trailing commas where valid in ES5 (objects, arrays, etc.). No trailing commas in type parameters in TypeScript.
 - `"none"` - No trailing commas.
-- `"all"` - Trailing commas wherever possible (including [trailing commas in function parameter lists and calls](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Trailing_commas#Trailing_commas_in_functions)). This requires node 8 or a modern browser that supports ES2017 or transform with [babel](https://babeljs.io/docs/en/index).
+- `"all"` - Trailing commas wherever possible (including [function parameters and calls](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Trailing_commas#Trailing_commas_in_functions)). To run, JavaScript code formatted this way needs an engine that supports ES2017 (Node.js 8+ or a modern browser) or [downlevel compilation](https://babeljs.io/docs/en/index). This also enables trailing commas in type parameters in TypeScript (supported since TypeScript 2.7 released in January 2018).
 
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