If we link to
```meson
[[#function]]('posarg')
```
then the ideal linkification would operate on "function" in the
formatted text:
```
function('posarg')
```
Instead, it operated on "function()" in the formatted text:
```
function()('posarg')
```
Fix this by detecting the next character after the reference, and
skipping the usual "automatically append the () for you" stage if it
already has that opening parenthesis.