Rollup merge of #69670 - GuillaumeGomez:explain-e0379, r=Dylan-DPC

Add explanation for E0379

r? @Dylan-DPC

Author: Dylan-DPC

src/librustc_error_codes/error_codes/E0379.md

@@ -1,3 +1,15 @@
+A trait method was declared const.
+
+Erroneous code example:
+
+```compile_fail,E0379
+#![feature(const_fn)]
+
+trait Foo {
+    const fn bar() -> u32; // error!
+}
+```
+
 Trait methods cannot be declared `const` by design. For more information, see
 [RFC 911].