Align advice to discourage issue-number-only names

https://rustc-dev-guide.rust-lang.org/tests/best-practices.html#test-naming specifically asks devs to "Avoid using only issue numbers as test names".

This commit introduces a minor change to https://rustc-dev-guide.rust-lang.org/tests/compiletest.html#crashes-tests that changes the advice to use only issue numbers as names to the advice to append them at the end of the file name.

Author: smanilov

src/tests/compiletest.md

@@ -565,8 +565,8 @@ $ COMPILETEST_VERBOSE_CRASHES=1 ./x test tests/crashes/999999.rs --stage 1
 ```
 
 When adding crashes from <https://github.com/rust-lang/rust/issues>, the issue
-number should be noted in the file name (`12345.rs` should suffice) and also
-inside the file include a `//@ known-bug: #4321` directive.
+number should be appended to the end of the file name (e.g. `calling-undefined-fn-12345.rs`).
+Also, include a `//@ known-bug: #12345` directive inside the file.
 
 If you happen to fix one of the crashes, please move it to a fitting
 subdirectory in `tests/ui` and give it a meaningful name. Please add a doc