zfs: add docs why we strip symbols manual

pkgs/os-specific/linux/zfs/default.nix

@@ -140,6 +140,9 @@ let
         "INSTALL_MOD_PATH=\${out}"
       ];
 
+      # Enabling BTF causes zfs to be build with debug symbols.
+      # Since zfs compress kernel modules on installation, our strip hooks skip stripping them.
+      # Hence we strip modules prior to compression.
       postBuild = optionalString buildKernel ''
          find . -name "*.ko" -print0 | xargs -0 -P$NIX_BUILD_CORES strip --strip-debug
       '';