Document thread safety.

Closes #196.

man/libzip.mdoc

diff --git a/man/libzip.mdoc b/man/libzip.mdoc
index dec548b..b3ade6b 100644
--- a/man/libzip.mdoc
+++ b/man/libzip.mdoc
@@ -29,7 +29,7 @@
 .\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
 .\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd July 23, 2020
+.Dd September 15, 2020
 .Dt LIBZIP 3
 .Os
 .Sh NAME
@@ -62,6 +62,19 @@
 .Pp
 Below there are two sections listing functions: one for how to read
 from zip archives and one for how to create/modify them.
+.Ss THREAD SAFETY
+In general, different zip archives opened by
+.Nm
+are independent of each other and can be used by parallel-running
+threads without locking.
+If you want to use an archive from multiple threads, you have to
+synchronize access to it yourself.
+If you use an archive as a source for
+.Xr zip_file_add 3
+or
+.Xr zip_file_replace 3 ,
+access to the target archive must be synchronized with access to the
+source archive as well.
 .Sh READING ZIP ARCHIVES
 .Ss open archive
 .Bl -bullet -compact