docs: specify that flake.lock files are JSON (#11594)

* docs: specify that flake.lock files are JSON

Recently, I decided that I was going to write some code that would parse
flake.lock files. I went to the Nix Reference Manual in order to look up
information on the format of flake.lock files, and I realized that a key
detail was missing from the Nix Reference Manual: it never says that
flake.lock files are JSON files. This commit fixes that issue.

This commit makes sure to specify that flake.lock files are encoded in
UTF-8. Confusingly, there’s multiple different JSON standards. Neither
ECMA-404, 2nd Edition [1] nor ISO/IEC 21778:2017 [2] mention UTF-8. RFC
8259 requires UTF-8, but only sometimes [3]. I chose to explicitly
specify that flake.lock files are UTF-8 in order to avoid any possible
ambiguities from the JSON standards.

[1]: <https://ecma-international.org/publications-and-standards/standards/ecma-404>
[2]: <https://www.iso.org/standard/71616.html>
[3]: <https://www.rfc-editor.org/rfc/rfc8259.html#section-8.1>

src/nix/flake.md

@@ -565,8 +565,9 @@ or NixOS modules, which are composed into the top-level flake's
 Inputs specified in `flake.nix` are typically "unlocked" in the sense
 that they don't specify an exact revision. To ensure reproducibility,
 Nix will automatically generate and use a *lock file* called
-`flake.lock` in the flake's directory. The lock file contains a graph
-structure isomorphic to the graph of dependencies of the root
+`flake.lock` in the flake's directory.
+The lock file is a UTF-8 JSON file.
+It contains a graph structure isomorphic to the graph of dependencies of the root
 flake. Each node in the graph (except the root node) maps the
 (usually) unlocked input specifications in `flake.nix` to locked input
 specifications. Each node also contains some metadata, such as the