show php function documentation upon completion

during completion, we rely on phpactor's output to show extra information (such as FQN) for each completion candidate, using company's `annotation` function.

company provides other ways to give contextual help for each candidate. Here's an extract from its source

> meta: The second argument is a completion candidate. Return a (short)
documentation string for it.

> doc-buffer: The second argument is a completion candidate. Return a
buffer with documentation for it. Preferably use `company-doc-buffer'. If
not all buffer contents pertain to this candidate, return a cons of buffer
and window start position.

It would be nice to use php's documentation when completion candidates are php functions (maybe using php-mode's `php-search-local-documentation` function ?)