unstable-book: Add page for the abi_efiapi feature

src/doc/unstable-book/src/language-features/abi-efiapi.md

@@ -0,0 +1,23 @@
+# `abi_efiapi`
+
+The tracking issue for this feature is: [#65815]
+
+[#65815]: https://github.com/rust-lang/rust/issues/65815
+
+------------------------
+
+The `efiapi` calling convention can be used for defining a function with
+an ABI compatible with the UEFI Interfaces as defined in the [UEFI
+Specification].
+
+Example:
+
+```rust
+#![feature(abi_efiapi)]
+
+extern "efiapi" { fn f1(); }
+
+extern "efiapi" fn f2() { todo!() }
+```
+
+[UEFI Specification]: https://uefi.org/specs/UEFI/2.10/