write.2: explain the atomicity guarantees of the writes

Also provide the pointer to the latest POSIX standard that justifies the
requirement.

Reviewed by:	emaste
Discussed with:	rmacklem
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D53361

lib/libsys/write.2

@@ -132,6 +132,26 @@ and
 may write fewer bytes than requested;
 the return value must be noted,
 and the remainder of the operation should be retried when possible.
+.Sh ATOMICITY OF WRITES
+When operating on regular files on local file systems, the effects of
+.Fn write
+are atomic.
+As required by the POSIX standard,
+the
+.Fn read ,
+.Fn write ,
+and
+.Fn ftruncate
+functions and their variations are atomic with respect to
+each other on the file data and metadata for regular files.
+See for instance
+.St -p1003.1-2024
+Volume 2, Section 2.9.7 for more information.
+.Pp
+.Fx
+implements the requirement by taking
+a read/write range lock on the file byte range
+affected by the corresponding function.
 .Sh RETURN VALUES
 Upon successful completion the number of bytes which were written
 is returned.