man: fix advice regarding thread safety of libsystemd

man: fix advice regarding thread safety of libsystemd

The prohibition to move libsystemd objects between threads was added in
64a7ef8bc06b5dcfcd9f99ea10a43bde75c4370f ('man: be more explicit about thread
safety of sd_journal'). At the time, this was valid, because we were using the
mempool for allocation and it apparently didn't handle access from different
threads. Sadlly, the commit links to a bugzilla entry referenced in the commit
is not publicly visible anymore, so the details are murky. But we stopped using
the mempool in a5d8835c78112206bbf0812dd4cb471f803bfe88 ('mempool: only enable
mempool use when linked to libsystemd-shared.so'), with subsequent followup in
b01f31954f1c7c4601925173ae2638b572224e9a ('Turn mempool_enabled() into a weak
symbol'). The restriction added in the man page is not necessary since then.

The text in the man page was arguably incorrect in calling the code
"thread-agnostic". If the code does not support being touched from threads at
all and has global state to tied to the main thread, it is not "agnostic", but
just doesn't support threads.

(I'm looking into https://github.com/systemd/python-systemd/issues/143, and
with the current scheme, the python-systemd module and all python code using
libsystemd would be very hard to use. With the change to free-threaded python
in python3.13, i.e. the replacement of single Global Interpreter Lock by
locking on individual objects, this limitation would become even more
constraining.)