Add explicit-extern-abis feature section to the unstable book

Author: obeis

src/doc/unstable-book/src/language-features/explicit-extern-abis.md

@@ -0,0 +1,23 @@
+# `explicit_extern_abis`
+
+The tracking issue for this feature is: #134986
+
+------
+
+Disallow `extern` without an explicit ABI. We should write `extern "C"`
+(or another ABI) instead of just `extern`.
+
+By making the ABI explicit, it becomes much clearer that "C" is just one of the
+possible choices, rather than the "standard" way for external functions.
+Removing the default makes it easier to add a new ABI on equal footing as "C".
+
+```rust,editionfuture,compile_fail
+#![feature(explicit_extern_abis)]
+
+extern fn function1() {}  // ERROR extern declarations without an explicit ABI
+                          // are disallowed
+
+extern "C" fn function2() {} // compile
+
+extern "aapcs" fn function3() {} // compile
+```