ld.so.8: Document the --preload command-line option added in glibc 2.30

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man8/ld.so.8

@@ -192,6 +192,24 @@ are delimited by colons or spaces.
 .B \-\-list
 List all dependencies and how they are resolved.
 .TP
+.BR \-\-preload " \fIlist\fP (since glibc 2.30)"
+Preload the objects specified in
+.IR list .
+The objects in
+.I list
+are delimited by colons or spaces.
+The objects are preloaded as explained in the description of the
+.BR LD_PRELOAD
+environment variable below.
+.IP
+By contrast with
+.BR LD_PRELOAD ,
+the
+.BR \-\-preload
+option provides a way to perform preloading for a single executable
+without affecting preloading performed in any child process that executes
+a new program.
+.TP
 .B \-\-verify
 Verify that program is dynamically linked and this dynamic linker can handle
 it.
@@ -347,6 +365,23 @@ as described above in
 .\"
 .\" which will preload the libmod.so in 'lib' or 'lib64', using it
 .\" in preference to the version in '.'.
+.IP
+There are various methods of specifying libraries to be preloaded,
+and these are handled in the following order:
+.RS
+.IP (1) 4
+The
+.BR LD_PRELOAD
+environment variable.
+.IP (2)
+The
+.B \-\-preload
+command-line option when invoking the dynamic linker directly.
+.IP (3)
+The
+.I /etc/ld.so.preload
+file (described below).
+.RE
 .TP
 .BR LD_TRACE_LOADED_OBJECTS
 If set (to any value), causes the program to list its dynamic