[DOC] Example usage for --zjit-dump-hir

Learning about the HIR format is difficult if you can't get zjit to
generate it for code snippets. It's not obvious to newcomers that HIR
won't be generated unless the `zjit-call-threshold` is reached (default
30).

Author: swebb

doc/jit/zjit.md

@@ -306,6 +306,14 @@ A file called `zjit_exits_{pid}.dump` will be created in the same directory as `
 stackprof path/to/zjit_exits_{pid}.dump
 ```
 
+### Viewing HIR as text
+
+The compiled zjit HIR can be viewed as text using the `--zjit-dump-hir` option. However, HIR will only be generated if the `zjit-call-threshold` is reached (default 30). By setting the threshold to 1 you can easily view the HIR for code snippets such as `1 + 1`:
+
+```bash
+./miniruby --zjit --zjit-dump-hir --zjit-call-threshold=1 -e "1 + 1"
+```
+
 ### Viewing HIR in Iongraph
 
 Using `--zjit-dump-hir-iongraph` will dump all compiled functions into a directory named `/tmp/zjit-iongraph-{PROCESS_PID}`. Each file will be named `func_{ZJIT_FUNC_NAME}.json`. In order to use them in the Iongraph viewer, you'll need to use `jq` to collate them to a single file. An example invocation of `jq` is shown below for reference.