Document thread safety.
Closes #196.
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
