document error signalling through (void*)0 return code.

mmap/mmap_private.3

@@ -16,5 +16,7 @@ will not be written to disk.
 The file is unmapped by the operating system if the process terminates.
 It can also be manually unmapped by calling \fBmunmap\fR from
 <sys/mman.h>.
+
+If the file could not be opened or mapped, (void*)0 is returned.
 .SH "SEE ALSO"
 munmap(2)

mmap/mmap_read.3

@@ -13,5 +13,7 @@ and returns a pointer to the mapped file.
 The file is unmapped by the operating system if the process terminates.
 It can also be manually unmapped by calling \fBmunmap\fR from
 <sys/mman.h>.
+
+If the file could not be opened or mapped, (void*)0 is returned.
 .SH "SEE ALSO"
 munmap(2)

mmap/mmap_shared.3

@@ -20,5 +20,7 @@ It can also be manually unmapped by calling \fBmunmap\fR from
 
 To write changes to disk immediately, you can use msync from
 <sys/mman.h>.
+
+If the file could not be opened or mapped, (void*)0 is returned.
 .SH "SEE ALSO"
 munmap(2), msync(2)