ldconfig.8: Document file filter and symlink pattern expectations

Information gleaned from comments in glibc's elf/ldconfig.c

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

man8/ldconfig.8

@@ -93,6 +93,28 @@ option.
 .B ldconfig
 should normally be run by the superuser as it may require write
 permission on some root owned directories and files.
+.PP
+Note that
+.B ldconfig
+will only look at files that are named
+.I lib*.so*
+(for regular shared objects) or
+.I ld-*.so*
+(for the dynamic loader itsef).  Other files will be ignored.  Also,
+.B ldconfig
+expects a certain pattern to how the symlinks are set up, like this
+example, where the middle file
+.RB ( libfoo.so.1
+here) is the SONAME for the library:
+.PP
+.in +4n
+.EX
+libfoo.so -> libfoo.so.1 -> libfoo.so.1.12
+.EE
+.in
+.PP
+Failure to follow this pattern may result in compatibility issues
+after an upgrade.
 .SH OPTIONS
 .TP
 .BR \-c " \fIfmt\fP, " \-\-format=\fIfmt\fP