Fix typo and improve documentation for E0632

compiler/rustc_error_codes/src/error_codes/E0632.md

@@ -0,0 +1,25 @@
+An explicit generic argument was provided when calling a function that
+uses `impl Trait` in argument position.
+
+Erroneous code example:
+
+```compile_fail,E0632
+fn foo<T: Copy>(a: T, b: impl Clone) {}
+
+foo::<i32>(0i32, "abc".to_string());
+```
+
+Either all generic arguments should be inferred at the call site, or
+the function definition should use an explicit generic type parameter
+instead of `impl Trait`. Example:
+
+```
+fn foo<T: Copy>(a: T, b: impl Clone) {}
+fn bar<T: Copy, U: Clone>(a: T, b: U) {}
+
+foo(0i32, "abc".to_string());
+
+bar::<i32, String>(0i32, "abc".to_string());
+bar::<_, _>(0i32, "abc".to_string());
+bar(0i32, "abc".to_string());
+```