Add general note about NULL pointers as arguments.

man/libzip.mdoc

@@ -1,5 +1,5 @@
 .\" libzip.mdoc -- general overview of available functions
-.\" Copyright (C) 2005-2022 Dieter Baron and Thomas Klausner
+.\" Copyright (C) 2005-2024 Dieter Baron and Thomas Klausner
 .\"
 .\" This file is part of libzip, a library to manipulate ZIP archives.
 .\" The authors can be contacted at <[email protected]>
@@ -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 December 5, 2022
+.Dd August 21, 2024
 .Dt LIBZIP 3
 .Os
 .Sh NAME
@@ -62,6 +62,10 @@ zip archive atomically.
 .Pp
 Below there are two sections listing functions: one for how to read
 from zip archives and one for how to create/modify them.
+.Sh GENERAL NOTES
+Unless explicitly documented, functions should not be passed
+.Dv NULL
+pointers as arguments.
 .Sh FILE NAMES
 .Ss Encoding
 Names of files in the host file system are expected in UTF-8 encoding.