Improve `nix-collect-garbage` docs

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/manual/src/command-ref/nix-collect-garbage.md

@@ -1,6 +1,6 @@
 # Name
 
-`nix-collect-garbage` - delete unreachable store paths
+`nix-collect-garbage` - delete unreachable [store objects]
 
 # Synopsis
 
@@ -8,17 +8,57 @@
 
 # Description
 
-The command `nix-collect-garbage` is mostly an alias of [`nix-store
---gc`](@docroot@/command-ref/nix-store/gc.md), that is, it deletes all
-unreachable paths in the Nix store to clean up your system. However,
-it provides two additional options: `-d` (`--delete-old`), which
-deletes all old generations of all profiles in `/nix/var/nix/profiles`
-by invoking `nix-env --delete-generations old` on all profiles (of
-course, this makes rollbacks to previous configurations impossible);
-and `--delete-older-than` *period*, where period is a value such as
-`30d`, which deletes all generations older than the specified number
-of days in all profiles in `/nix/var/nix/profiles` (except for the
-generations that were active at that point in time).
+The command `nix-collect-garbage` is mostly an alias of [`nix-store --gc`](@docroot@/command-ref/nix-store/gc.md).
+That is, it deletes all unreachable [store objects] in the Nix store to clean up your system.
+
+However, it provides two additional options,
+[`--delete-old`](#opt-delete-old) and [`--delete-older-than`](#opt-delete-older-than),
+which also delete old [profiles], allowing potentially more [store objects] to be deleted because profiles are also garbage collection roots.
+These options are the equivalent of running
+[`nix-env --delete-generations`](@docroot@/command-ref/nix-env/delete-generations.md)
+with various augments on multiple profiles,
+prior to running `nix-collect-garbage` (or just `nix-store --gc`) without any flags.
+
+> **Note**
+>
+> Deleting previous configurations makes rollbacks to them impossible.
+
+These flags should be used with care, because they potentially delete generations of profiles used by other users on the system.
+
+## Locations searched for profiles
+
+`nix-collect-garbage` cannot know about all profiles; that information doesn't exist.
+Instead, it looks in a few locations, and acts on all profiles it finds there:
+
+1. The default profile locations as specified in the [profiles] section of the manual.
+
+2. > **NOTE**
+   >
+   > Not stable; subject to change
+   >
+   > Do not rely on this functionality; it just exists for migration purposes and is may change in the future.
+   > These deprecated paths remain a private implementation detail of Nix.
+
+   `$NIX_STATE_DIR/profiles` and `$NIX_STATE_DIR/profiles/per-user`.
+
+   With the exception of `$NIX_STATE_DIR/profiles/per-user/root` and `$NIX_STATE_DIR/profiles/default`, these directories are no longer used by other commands.
+   `nix-collect-garbage` looks there anyways in order to clean up profiles from older versions of Nix.
+
+# Options
+
+These options are for deleting old [profiles] prior to deleting unreachable [store objects].
+
+- <span id="opt-delete-old">[`--delete-old`](#opt-delete-old)</span> / `-d`\
+  Delete all old generations of profiles.
+
+  This is the equivalent of invoking `nix-env --delete-generations old` on each found profile.
+
+- <span id="opt-delete-older-than">[`--delete-older-than`](#opt-delete-older-than)</span> *period*\
+  Delete all generations of profiles older than the specified amount (except for the generations that were active at that point in time).
+  *period* is a value such as `30d`, which would mean 30 days.
+
+  This is the equivalent of invoking [`nix-env --delete-generations <period>`](@docroot@/command-ref/nix-env/delete-generations.md#generations-days) on each found profile.
+  See the documentation of that command for additional information about the *period* argument.
 
 {{#include ./opt-common.md}}
 
@@ -32,3 +72,6 @@ generations of each profile, do
 ```console
 $ nix-collect-garbage -d
 ```
+
+[profiles]: @docroot@/command-ref/files/profiles.md
+[store objects]: @docroot@/glossary.md#gloss-store-object