ld-libs: Make it clearer that /lib:/usr/lib is only a fallback
glibc searches /lib:/usr/lib as a hard-coded fallback, but we don't expect to get there when searching for dependencies in practice: on OSs that have dedicated 32- and 64-bit directories (such as Red-Hat-style multilib, Arch-style multilib and Debian-style multiarch), those directories are searched indirectly as part of ld.so.cache.
Mentioning that this is a fallback makes it a little bit more obvious that it isn't an error that we were not searching /usr/lib32, /usr/lib64 or /usr/lib/MULTIARCH when looking for a library of the appropriate word size.
While I'm here, deduplicate the end of the message a bit.
Suggested-by: @evelikov