Support for variadic functions in documentation popup.

Summary:
This shows the documentation for functions defined as

function foo(...$params) {}
function foo(A ...$params) {}

in the form of

void foo ([mixed ...params])
void foo ([A ...params])

In the first case, $params itself is shown as

array params

and in the latter as

array of (A) params

Reviewers: zhigalin

Reviewed By: zhigalin

Subscribers: aacid, zhigalin, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6256