Address comments; also handled nested .envrcs via source_up_if_exists

Author: VVKot

.envrc

@@ -1,2 +1,5 @@
-use flake
+source_up_if_exists
+if command -v nix >/dev/null 2>&1; then
+  use flake
+fi
 source_env_if_exists .envrc.local

docs/source/contributor-guide/development_environment.md

@@ -58,9 +58,14 @@ The repository's `flake.nix` provides a Nix development shell. Enter it directly
 with `nix develop`, or install and configure [direnv](https://direnv.net/) and run
 `direnv allow` from the repository root to load it automatically.
 
-Put machine-specific direnv settings in `.envrc.local`. This file is ignored by
-Git and loaded after the shared Nix environment so that local settings take
-precedence.
+The repository's `.envrc` first loads the nearest ancestor `.envrc`, if one
+exists, and then loads the shared Nix environment when Nix is installed. Put
+project-specific direnv settings in `.envrc.local`. This file is ignored by Git
+and loaded last so that local settings take precedence.
+
+If you previously kept personal settings in the repository's `.envrc`, move
+only those settings to `.envrc.local`. Do not copy the shared `.envrc`, as doing
+so would load the Nix environment twice.
 
 ## Windows Setup