Add test of fn pointer without argument name

Author: dtolnay

Diff for: tests/rustdoc/fn-pointer-arg-name.rs

@@ -3,3 +3,11 @@
 //@ has foo/fn.f.html
 //@ has - '//pre[@class="rust item-decl"]' 'pub fn f(callback: fn(len: usize, foo: u32))'
 pub fn f(callback: fn(len: usize, foo: u32)) {}
+
+//@ has foo/fn.g.html
+//@ has - '//pre[@class="rust item-decl"]' 'pub fn g(_: fn(_: usize, _: u32))'
+pub fn g(_: fn(usize, _: u32)) {}
+
+//@ has foo/fn.mixed.html
+//@ has - '//pre[@class="rust item-decl"]' 'pub fn mixed(_: fn(_: usize, foo: u32))'
+pub fn mixed(_: fn(usize, foo: u32)) {}