nixos/nscd: Add a descriptive comment to the nscd configuration

nixos/modules/services/system/nscd.conf

@@ -1,3 +1,11 @@
+# We basically use nscd as a proxy for forwarding nss requests to appropriate
+# nss modules, as we run nscd with LD_LIBRARY_PATH set to the directory
+# containing all such modules
+# Note that we can not use `enable-cache no` As this will actually cause nscd
+# to just reject the nss requests it receives, which then causes glibc to
+# fallback to trying to handle the request by itself. Which won't work as glibc
+# is not aware of the path in which the nss modules live.  As a workaround, we
+# have `enable-cache yes` with an explicit ttl of 0
 server-user             nscd
 threads                 1
 paranoia                no