reword documentation on nix-copy-closure (#10709)

* reword documentation on `nix-copy-closure`

- one sentence per line
- be more precise with respect to which Nix stores are being accessed
- make a clear distinction between store paths and store objects
- add links to definitions of terms
- clarify which machine is which
- --to and --from don't take arguments

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

doc/manual/src/command-ref/nix-copy-closure.md

@@ -1,75 +1,64 @@
 # Name
 
-`nix-copy-closure` - copy a closure to or from a remote machine via SSH
+`nix-copy-closure` - copy store objects to or from a remote machine via SSH
 
 # Synopsis
 
 `nix-copy-closure`
-  [`--to` | `--from`]
+  [`--to` | `--from` ]
   [`--gzip`]
   [`--include-outputs`]
   [`--use-substitutes` | `-s`]
   [`-v`]
-  _user@machine_ _paths_
+  [_user_@]_machine_[:_port_] _paths_
 
 # Description
 
-`nix-copy-closure` gives you an easy and efficient way to exchange
-software between machines.  Given one or more Nix store _paths_ on the
-local machine, `nix-copy-closure` computes the closure of those paths
-(i.e. all their dependencies in the Nix store), and copies all paths
-in the closure to the remote machine via the `ssh` (Secure Shell)
-command.  With the `--from` option, the direction is reversed: the
-closure of _paths_ on a remote machine is copied to the Nix store on
-the local machine.
+Given _paths_ from one machine, `nix-copy-closure` computes the [closure](@docroot@/glossary.md#gloss-closure) of those paths (i.e. all their dependencies in the Nix store), and copies [store objects](@docroot@/glossary.md#gloss-store-object) in that closure to another machine via SSH.
+It doesn’t copy store objects that are already present on the other machine.
 
-This command is efficient because it only sends the store paths
-that are missing on the target machine.
+> **Note**
+>
+> While the Nix store to use on the local machine can be specified on the command line with the [`--store`](@docroot@/command-ref/conf-file.md#conf-store) option, the Nix store to be accessed on the remote machine can only be [configured statically](@docroot@/command-ref/conf-file.md#configuration-file) on that remote machine.
 
-Since `nix-copy-closure` calls `ssh`, you may be asked to type in the
-appropriate password or passphrase.  In fact, you may be asked _twice_
-because `nix-copy-closure` currently connects twice to the remote
-machine, first to get the set of paths missing on the target machine,
-and second to send the dump of those paths.  When using public key
-authentication, you can avoid typing the passphrase with `ssh-agent`.
+Since `nix-copy-closure` calls `ssh`, you may need to authenticate with the remote machine.
+In fact, you may be asked for authentication _twice_ because `nix-copy-closure` currently connects twice to the remote machine: first to get the set of paths missing on the target machine, and second to send the dump of those paths.
+When using public key authentication, you can avoid typing the passphrase with `ssh-agent`.
 
 # Options
 
-  - `--to`\
-    Copy the closure of _paths_ from the local Nix store to the Nix
-    store on _machine_. This is the default.
+  - `--to`
 
-  - `--from`\
-    Copy the closure of _paths_ from the Nix store on _machine_ to the
-    local Nix store.
+    Copy the closure of _paths_ from a Nix store accessible from the local machine to the Nix store on the remote _machine_.
+    This is the default behavior.
+
+  - `--from`
+
+    Copy the closure of _paths_ from the Nix store on the remote _machine_ to the local machine's specified Nix store.
+
+  - `--gzip`
 
-  - `--gzip`\
     Enable compression of the SSH connection.
 
-  - `--include-outputs`\
+  - `--include-outputs`
+
     Also copy the outputs of [store derivation]s included in the closure.
 
     [store derivation]: @docroot@/glossary.md#gloss-store-derivation
 
-  - `--use-substitutes` / `-s`\
-    Attempt to download missing paths on the target machine using Nix’s
-    substitute mechanism.  Any paths that cannot be substituted on the
-    target are still copied normally from the source.  This is useful,
-    for instance, if the connection between the source and target
-    machine is slow, but the connection between the target machine and
-    `nixos.org` (the default binary cache server) is
-    fast.
+  - `--use-substitutes` / `-s`
 
-  - `-v`\
-    Show verbose output.
+    Attempt to download missing store objects on the target from [substituters](@docroot@/command-ref/conf-file.md#conf-substituters).
+    Any store objects that cannot be substituted on the target are still copied normally from the source.
+    This is useful, for instance, if the connection between the source and target machine is slow, but the connection between the target machine and `cache.nixos.org` (the default binary cache server) is fast.
 
 {{#include ./opt-common.md}}
 
 # Environment variables
 
-  - `NIX_SSHOPTS`\
-    Additional options to be passed to `ssh` on the command
-    line.
+  - `NIX_SSHOPTS`
+
+    Additional options to be passed to `ssh` on the command line.
 
 {{#include ./env-common.md}}