Fix documentation of xcb_poll_for_event()

Fix documentation of xcb_poll_for_event()

In commit 8eba8690adac2, the API documentation for xcb_poll_for_event() was
fixed to remove an argument that was previously removed in commit 34168ab549.

However, that commit only removed the first line of the documentation, leaving
behind a spurious half-sentence. That commit happened seven years ago and now
finally someone noticed...

Thanks to Benjamin Herr for reporting this on IRC.

v2: Thanks again to Benjamin Herr for noticing that my commit message blamed the
wrong commit.

Signed-off-by: Uli Schlachter <psychon@znc.in>