threads::shared: Remove redundant description just added

threads::shared: Remove redundant description just added

In commit fd013656aac I moved the documentation for the two warnings
that threads::shared produces from perldiag.pod to shared.pm, without
rewording anything.

These two warnings had almost identical descriptions, and--I found
later--basically repeat some of the documentation of the cond_signal
function, so point to that description in the warnings section and
avoid repeating things.