Keyword becoming a paragraph based on optval

[Tom Alexander]

Emacs version: 29.1
Org-mode version: f3de4c3e041e0ea825b5b512dc0db37c78b7909e (latest in git)

This test document parses as a keyword:
```
#+CAPTION[*foo*]: baz
```

but this test document parses as a paragraph:
```
#+CAPTION[*foo* bar]: baz
```

--
Tom Alexander
pgp: https://fizz.buzz/pgp.asc