Update supported values of target_os

'win32' -> 'windows', added 'ios'

Author: vadimcn

src/doc/reference.md

@@ -2146,7 +2146,7 @@ The following configurations must be defined by the implementation:
   `"unix"` or `"windows"`. The value of this configuration option is defined
   as a configuration itself, like `unix` or `windows`.
 * `target_os = "..."`. Operating system of the target, examples include
-  `"win32"`, `"macos"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
+  `"windows"`, `"macos"`, `"ios"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
   `"bitrig"` or `"openbsd"`.
 * `target_pointer_width = "..."`. Target pointer width in bits. This is set
   to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for