Update default.conf

In default.conf explain how to use LIBS properly,
see #2743

usr/share/rear/conf/default.conf

@@ -1484,7 +1484,27 @@ uniq
 wc
 )
 
-# Library files to include in the rescue/recovery system:
+# Special library files to include in the rescue/recovery system:
+# Needed programs in the recovery system should be specified via
+# PROGS+=( non_mandatory_program )
+# REQUIRED_PROGS+=( mandatory_program )
+# Normally there is no need to specify LIBS for programs
+# because needed libraries for "normally" linked programs
+# get automatically included via the RequiredSharedObjects function
+# which calls ldd to determine the required shared objects
+# of the elements in the PROGS and REQUIRED_PROGS arrays
+# (cf. COPY_AS_IS and TRUSTED_FILE_OWNERS below).
+# Therefore only libraries that are not reported by ldd
+# (e.g. when libraries are loaded via dlopen by a program)
+# must be specified in LIBS.
+# RequiredSharedObjects() is also called for the elements in LIBS
+# but it does not descend recursively into (sub)-directories
+# so the elements in LIBS must evaluate to actual library files
+# (by bash pathname expansion with 'nullglob' that is set in ReaR).
+# For example instead of using only LIBS+=( /path/to/libdir )
+# specify LIBS+=( /path/to/libdir/*.so* /path/to/libdir/*/*.so* )
+# for library files in /path/to/libdir and in its direct sub-directories
+# cf. https://github.com/rear/rear/issues/2743
 LIBS=()
 
 # Kernel modules to include in the rescue/recovery system: