Add long explanation for E0757

compiler/rustc_error_codes/src/error_codes/E0757.md

@@ -0,0 +1,25 @@
+A function was given both the `ffi_const` and `ffi_pure` attributes.
+
+Erroneous code example:
+
+```compile_fail,E0757
+#![feature(ffi_const, ffi_pure)]
+
+extern "C" {
+    #[ffi_const]
+    #[ffi_pure] // error: `#[ffi_const]` function cannot be `#[ffi_pure]`
+    pub fn square(num: i32) -> i32;
+}
+```
+
+As `const` has a stricter set of requirements than `pure`, remove the `ffi_pure`
+attribute:
+
+```
+#![feature(ffi_const)]
+
+extern "C" {
+    #[ffi_const]
+    pub fn square(num: i32) -> i32;
+}
+```