Fix some dangling references

doc/manual/src/package-management/basic-package-mgmt.md

@@ -24,11 +24,11 @@ or completely new ones.)
 
 You can manually download the latest version of Nixpkgs from
 <http://nixos.org/nixpkgs/download.html>. However, it’s much more
-convenient to use the Nixpkgs *channel*, since it makes it easy to stay
-up to date with new versions of Nixpkgs. (Channels are described in more
-detail in [???](#sec-channels).) Nixpkgs is automatically added to your
-list of “subscribed” channels when you install Nix. If this is not the
-case for some reason, you can add it as follows:
+convenient to use the Nixpkgs [*channel*](channels.md), since it makes
+it easy to stay up to date with new versions of Nixpkgs. Nixpkgs is
+automatically added to your list of “subscribed” channels when you
+install Nix. If this is not the case for some reason, you can add it
+as follows:
 
     $ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
     $ nix-channel --update

doc/manual/src/package-management/copy-closure.md

@@ -8,7 +8,7 @@ dependencies:
 
     $ nix-copy-closure --to alice@itchy.example.org $(type -p firefox)
 
-See [???](#sec-nix-copy-closure) for details.
+See the [manpage for `nix-copy-closure`](../command-ref/nix-copy-closure.md) for details.
 
 With `nix-store
 --export` and `nix-store --import` you can write the closure of a store

doc/manual/src/package-management/package-management.md

@@ -1,4 +1,5 @@
-This chapter discusses how to do package management with Nix, i.e., how
-to obtain, install, upgrade, and erase packages. This is the “user’s”
-perspective of the Nix system — people who want to *create* packages
-should consult [???](#chap-writing-nix-expressions).
+This chapter discusses how to do package management with Nix, i.e.,
+how to obtain, install, upgrade, and erase packages. This is the
+“user’s” perspective of the Nix system — people who want to *create*
+packages should consult the [chapter on writing Nix
+expressions](../expressions/writing-nix-expressions.md).

doc/manual/src/package-management/profiles.md

@@ -104,8 +104,7 @@ These commands switch to the `my-profile` and default profile,
 respectively. If the profile doesn’t exist, it will be created
 automatically. You should be careful about storing a profile in another
 location than the `profiles` directory, since otherwise it might not be
-used as a root of the garbage collector (see
-[???](#sec-garbage-collection)).
+used as a root of the [garbage collector](garbage-collection.md).
 
 All `nix-env` operations work on the profile pointed to by
 `~/.nix-profile`, but you can override this using the `--profile` option