Rename doc/manual{src -> source}

This is needed to avoid this
https://github.com/mesonbuild/meson/issues/13774 when we go back to
making our subproject directory `src`.

doc/manual/source/command-ref/nix-store/opt-common.md

@@ -0,0 +1,36 @@
+# Options
+
+The following options are allowed for all `nix-store` operations, but may not always have an effect.
+
+- <span id="opt-add-root">[`--add-root`](#opt-add-root)</span> *path*
+
+  Causes the result of a realisation (`--realise` and
+  `--force-realise`) to be registered as a root of the garbage
+  collector. *path* will be created as a symlink to the resulting
+  store path. In addition, a uniquely named symlink to *path* will
+  be created in `/nix/var/nix/gcroots/auto/`. For instance,
+
+  ```console
+  $ nix-store --add-root /home/eelco/bla/result --realise ...
+
+  $ ls -l /nix/var/nix/gcroots/auto
+  lrwxrwxrwx    1 ... 2005-03-13 21:10 dn54lcypm8f8... -> /home/eelco/bla/result
+
+  $ ls -l /home/eelco/bla/result
+  lrwxrwxrwx    1 ... 2005-03-13 21:10 /home/eelco/bla/result -> /nix/store/1r11343n6qd4...-f-spot-0.0.10
+  ```
+
+  Thus, when `/home/eelco/bla/result` is removed, the GC root in the
+  `auto` directory becomes a dangling symlink and will be ignored by
+  the collector.
+
+  > **Warning**
+  >
+  > Note that it is not possible to move or rename GC roots, since
+  > the symlink in the `auto` directory will still point to the old
+  > location.
+
+  If there are multiple results, then multiple symlinks will be
+  created by sequentially numbering symlinks beyond the first one
+  (e.g., `foo`, `foo-2`, `foo-3`, and so on).
+