[c-interop][docs] Add @_extern(c) attribute explanation

Author: kateinoigakukun

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -449,6 +449,20 @@ through WebAssembly's import interface.
 
 It's the equivalent of clang's `__attribute__((import_module("module"), import_name("field")))`.
 
+### `@_extern(c, [, <"cName">])`
+
+Indicates that a particular declaration should refer to a
+C declaration with the given name. If the optional "cName"
+string is not specified, the mangled Swift name will be used.
+
+Similar to `@_cdecl`, but this attribute is used to reference
+C declarations from Swift, while `@_cdecl` is used to be referenced
+from C.
+
+Also similar to `@_silgen_name`, but a function declared with
+`@_extern(c)` is assumed to use the C ABI, while `@_silgen_name`
+assumes the Swift ABI.
+
 ## `@_fixed_layout`
 
 Same as `@frozen` but also works for classes.