Add details about unsafe_op_in_unsafe_fn to E0133

Author: Nemo157

Diff for: compiler/rustc_error_codes/src/error_codes/E0133.md

@@ -1,4 +1,4 @@
-Unsafe code was used outside of an unsafe function or block.
+Unsafe code was used outside of an unsafe block.
 
 Erroneous code example:
 
@@ -30,4 +30,21 @@ fn main() {
 
 See the [unsafe section][unsafe-section] of the Book for more details.
 
+#### Unsafe code in functions
+
+Unsafe code is currently accepted in unsafe functions, but that is being phased
+out in favor of requiring unsafe blocks here too.
+
+```
+unsafe fn f() { return; }
+
+unsafe fn g() {
+    f(); // Is accepted, but no longer recommended
+    unsafe { f(); } // Recommended way to write this
+}
+```
+
+Linting against this is controlled via the `unsafe_op_in_unsafe_fn` lint, which
+is `allow` by default but will be upgraded to `warn` in a future edition.
+
 [unsafe-section]: https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html