Add E0727 long explanation

Author: LeSeulArtichaut

src/librustc_error_codes/error_codes.rs

@@ -397,6 +397,7 @@ E0718: include_str!("./error_codes/E0718.md"),
 E0720: include_str!("./error_codes/E0720.md"),
 E0723: include_str!("./error_codes/E0723.md"),
 E0725: include_str!("./error_codes/E0725.md"),
+E0727: include_str!("./error_codes/E0727.md"),
 E0728: include_str!("./error_codes/E0728.md"),
 E0729: include_str!("./error_codes/E0729.md"),
 E0730: include_str!("./error_codes/E0730.md"),
@@ -607,6 +608,5 @@ E0746: include_str!("./error_codes/E0746.md"),
     E0722, // Malformed `#[optimize]` attribute
     E0724, // `#[ffi_returns_twice]` is only allowed in foreign functions
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
-    E0727, // `async` generators are not yet supported
     E0739, // invalid track_caller application/syntax
 }

src/librustc_error_codes/error_codes/E0727.md

@@ -0,0 +1,26 @@
+A `yield` clause was used in an `async` context.
+
+Example of erroneous code:
+
+```compile_fail
+#![feature(generators)]
+
+let generator = || {
+    async {
+        yield;
+    }
+};
+```
+
+Here, the `yield` keyword is used in an `async` block,
+which is not yet supported.
+
+To fix this error, you have to move `yield` out of the `async` block:
+
+```
+#![feature(generators)]
+
+let generator = || {
+    yield;
+};
+```