Add 'nix flake' manpages

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diff --git a/src/nix/flake-update.md b/src/nix/flake-update.md
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+R""(
+
+# Examples
+
+* Update the `nixpkgs` and `nix` inputs of the flake in the current
+  directory:
+
+  ```console
+  # nix flake update --update-input nixpkgs --update-input nix
+  * Updated 'nix': 'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' -> 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c'
+  * Updated 'nixpkgs': 'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' -> 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293'
+  ```
+
+* Recreate the lock file (i.e. update all inputs) and commit the new
+  lock file:
+
+  ```console
+  # nix flake update --recreate-lock-file --commit-lock-file
+  …
+  warning: committed new revision '158bcbd9d6cc08ab859c0810186c1beebc982aad'
+  ```
+
+# Description
+
+This command updates the lock file of a flake (`flake.lock`) so that
+it contains a lock for every flake input specified in
+`flake.nix`. Note that every command that operates on a flake will
+also update the lock file if needed, and supports the same
+flags. Therefore,
+
+```console
+# nix flake update --update-input nixpkgs
+# nix build
+```
+
+is equivalent to:
+
+```console
+# nix build --update-input nixpkgs
+```
+
+Thus, this command is only useful if you want to update the lock file
+separately from any other action such as building.
+
+> **Note**
+>
+> This command does *not* update locks that are already present unless
+> you explicitly ask for it using `--update-input` or
+> `--recreate-lock-file`. Thus, if the lock file already has locks for
+> every input, then `nix flake update` (without arguments) does
+> nothing.
+
+)""