Update guide for forall proxy recommendation (#439)

Author: JordanMartinez

migration-guides/0.15-Migration-Guide.md

@@ -427,9 +427,10 @@ case foo of
 ### Changes affecting multiple libraries
 
 - Migrated all FFI to ES modules and dropped support for CommonJS modules.
+- Removed deprecated `MonadZero` type class and all of its deprecated instances.
 - Removed all kind-specific Proxy types (e.g. `SProxy`, `Proxy2`, `Proxy3`, `RLProxy`, etc.)
     - Replace usage of such types with `Type.Proxy (Proxy(..))`.
-- Removed deprecated `MonadZero` type class and all of its deprecated instances.
+    - See the recommendation about updating the `forall proxy.` workaround at the bottom of this file.
 
 ### Removal of deprecated things and their replacements
 
@@ -848,6 +849,39 @@ These three functions were defined in `purescript-web-dom` and duplicated in the
 
 ## Other Recommendations
 
+### Removing the `forall proxy.` workaround
+
+In `0.13.x`, we did not have a kind-polymorphic `Proxy` type (e.g. `data Proxy :: forall k. k -> Type`). So, each kind needed its own `Proxy` type, producing a zoo of such types:
+- `SProxy` - `Proxy` for kind `Symbol`
+- `RProxy` - `Proxy` for kind `Row`
+- `RLProxy` - `Proxy` for kind `RowList`
+- ... etc.
+
+```purs
+-- 0.13.x
+foo :: forall sym. IsSymbol sym => SProxy sym -> ...
+
+bar :: forall row. RowCons "foo" Int () row => RProxy row -> ...
+```
+
+In `0.14.0`, when polykinds were implemented, we got a kind-generic `Proxy` type. Thus, the zoo of `Proxy` types were no longer needed. However, to reduce breakage, we used a `forall proxy.` workaround so that both the kind-specific and kind-generic version worked.
+
+```purs
+-- 0.14.x
+-- `forall proxy.` workaround:
+foo :: forall proxy sym. IsSymbol sym => proxy sym -> ...
+
+foo (SProxy :: SProxy "a") -- kind-specific Proxy type works!
+foo (Proxy :: Proxy "a")   -- kind-generic Proxy type also works!
+```
+
+In `0.15.x`, the zoo of `Proxy` types were removed. **Thus, the `forall proxy.` workaround SHOULD be removed in all libraries as doing so improves type inference.**
+```diff
+-foo :: forall proxy sym. IsSymbol sym => proxy sym -> ...
++import Type.Proxy (Proxy(..))
++foo :: forall sym. IsSymbol sym => Proxy sym -> ...
+```
+
 ### Discouraging usage of `F` and `FT`
 
 **To fix:**