Misc doc updates

Author: chenglou

pages/docs/manual/latest/api/js/obj.mdx

@@ -51,7 +51,7 @@ Js.log(target)
 ## keys
 
 ```res sig
-let keys: Js.t<'a> => array<string>
+let keys: {..} => array<string>
 ```
 
 `keys(obj)` returns an `array` of the keys of `obj`'s own enumerable properties.

pages/docs/manual/latest/import-from-export-to-js.mdx

@@ -76,7 +76,7 @@ var paddedResult = LeftPad("hi", 5);
 
 Depending on whether you're compiling ReScript to CommonJS or ES6 module, **this feature will generate subtly different code**. Please check both output tabs to see the difference. The ES6 output here would be wrong!
 
-### Import an ES6 Default Export (Since >= 8.3.0)
+### Import an ES6 Default Export
 
 Use the value `"default"` on the right hand side:
 

pages/docs/manual/latest/migrate-from-bucklescript-reason.mdx

@@ -56,7 +56,7 @@ npx rescript convert -all
 - First class module: from `(module S: Student)` to `module(S: Student)`.
 - No custom infix operator for now (including `mod`).
 - Object access: from `settings##visible #= true` to `settings["visible"] = true`. Rejoice!
-- Object: from `Js.t({"age": int})` to just `{"age": int}`. The `Js.t` part is now implicit.
+- Object: from `Js.t({"age": int})` to just `{"age": int}`. The `Js.t` part is now uneeded.
 - Attribute: from `[@myAttribute "hello"]` to `@myAttribute("hello")`. From `[%re bla]` to `%re(bla)`.
 - Removed dereference syntax `result^`. Just use `result.contents`.
 - `fun` pattern matching syntax removed.

pages/docs/manual/latest/pattern-matching-destructuring.mdx

@@ -747,7 +747,7 @@ Here are some advices.
 
 Do not abuse the wildcard `_` too much. This prevents the compiler from giving you better exhaustiveness check, which would be especially important after a refactoring where you add a new case to a variant. Try only using `_` against infinite possibilities, e.g. string, int, etc.
 
-Use `when` clause sparingly.
+Use `if` clause sparingly.
 
 **Flatten your pattern-match whenever you can**. This is a real bug remover. Here's a series of examples, from worst to best:
 

pages/docs/manual/latest/reserved-keywords.mdx

@@ -24,7 +24,6 @@ canonical: "/docs/manual/latest/reserved-keywords"
 
 <!-- - `do` -->
 <!-- - `done` -->
-- `downto`
 
 
 - `else`
@@ -79,7 +78,6 @@ canonical: "/docs/manual/latest/reserved-keywords"
 
 
 <!-- - `then` -->
-- `to`
 - `true`
 - `try`
 - `type`

pages/docs/manual/latest/shared-data-types.mdx

@@ -22,15 +22,14 @@ Unlike most compiled-to-js languages, in ReScript, **you don't need to write dat
 - Object. ReScript objects are JavaScript objects, vice-versa.
 - Function. They compile to clean JS functions.
 - Module. ReScript files are considered top-level modules, and are compiled to JS files 1 to 1. Nested modules are compiled to JavaScript objects.
-- Unit. The `unit` type, which has a single value `()`, compiles to `undefined` too. Likewise, you can treat an incoming JS `undefined` as `()`.
-
-<!-- TODO: talk about poly variant -->
+- Polymorphic variants.
+- Unit. The `unit` type, which has a single value `()`, compiles to `undefined` too. Likewise, you can treat an incoming JS `undefined` as `()` if that's the only value it'll ever be.
 
 **Types that are slightly different than JS, but that you can still use from JS**:
 - Int. **Ints are 32-bits**! Be careful, you can potentially treat them as JS numbers and vice-versa, but if the number's large, then you better treat JS numbers as floats. For example, we bind to Js.Date using `float`s.
 - Option. The `option` type's `None` value compiles into JS `undefined`. The `Some` value, e.g. `Some(5)`, compiles to `5`. Likewise, you can treat an incoming JS `undefined` as `None`. **JS `null` isn't handled here**. If your JS value can be `null`, use [Js.Nullable](api/js/nullable) helpers.
 - Exception.
-- Variant. Check the compiled JavaScript output of variant to see its shape. We don't recommend exporting a ReScript variant for pure JS usage, but you can do that if you have some interop needs.
+- Variant. Check the compiled JavaScript output of variant to see its shape. We don't recommend exporting a ReScript variant for pure JS usage, since they're harder to read as plain JS code, but you can do it.
 - List, which is just a regular variant.
 
 **Non-shared types (aka internal types)**: