manual: Remove warning in the documentation of the abort function

The warning is confusing to those who do not understand the context,
and the warning is easy to misunderstand:

A reader needs to know that it was written by someone who is generally
skeptical of government influence and control, otherwise it reads as
an affirmation of the U.S. government's role as the ultimate editor of
the manual.  This is precisely the opposite of what the warning
intends to convey.  (Reportedly, it criticizes that several
U.S. administrations have tried to restrict the medical advice that
U.S.-funded health care workers can provide abroad, considering that
censorship.)

The warning is also misleading on a technical level.  A reader who
makes the connection to pregnancy termination will get the wrong
impression that calling the abort function will terminate subprocesses
of the current process, but this is not what generally happens.

Finally, for both GNU and the FSF, it is inappropriate to use female
reproductive health as mere joke material, since these organizations
do not concern themselves with such issues otherwise, and the warning
is purportedly about something else entirely.

This reinstates commit 340d9652
("manual/startup.texi (Aborting a Program): Remove inappropriate
joke."), effectively reverting the revert in commit
ffa81c22 ("Revert:").

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>