Document the `ignore` function attribute.
This is very brief, without any usage or motivation. There's more
information in the 'Testing' section of the Rust Book.
@@ -93,6 +93,7 @@ type int8_t = i8;
item](#language-items) for more details.
- `test` - indicates that this function is a test function, to only be compiled
in case of `--test`.
+ - `ignore` - indicates that this test function is disabled.
- `should_panic` - indicates that this test function should panic, inverting the success condition.
- `cold` - The function is unlikely to be executed, so optimize it (and calls
to it) differently.
