fetch: Stop setting an alarm

Now that fetchTimeout works reliably, setting an alarm is not only no
longer necessary but counterproductive, as it will trigger even if the
connection is not actually stalled but merely slow.

While here, improve the wording of the manual page's description of the
various options for setting a timeout.

MFC after: 1 week
Reviewed by: op
Differential Revision: https://reviews.freebsd.org/D57911

(cherry picked from commit 3dddfe29248c47d1a80dc96a76a308ae910b2a24)