internal link button supplies bad template
The toolbar is served using MediaWiki core, and the text is contained within the message i18n files that come with the installation.
1. We don't edit the core files as it's impossible to keep a track of all the changes when we upgrade 2. If we change the english text for that, we also need to change the 385 other languages. Then we need to manually patch each language file every time we upgrade.
So primarily because of point 1, unfortunately, this cannot be fixed.
I guessed that it was probably pretty core. Maybe an enhancement request to MediaWiki pointing out the ambiguity in this buttons functionality then would be good. I reckon it should say [[<link title>]] rather than [[link title]]. Because as it stands it is not consistent with what the external link button offers up. which is [http://www.example.com link title] So for consistency the external link one could actually be [<http://www.example.com> <link title>]
or maybe [http://www.example.com LinkTitle] and [[LinkTitle]] But perhaps this is a discussion to by had on the MediaWiki web site not this one.
I will shut up now.