Vimdoc should write optional parameters as [arg='default'] in function headers

[Soares]

This is a little nicer than just tossing "[foo] is value by default" at the end of the doc paragraphs.

[dbarnett]

…but would confuse vim's help file syntax highlighting. And if the default is a strange literal expression, like [1, 2, 3], or an English phrase like "the file being edited", it could look pretty weird. We may or may not actually want to do this.