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-env/delete-generations.md

@@ -0,0 +1,98 @@
+# Name
+
+`nix-env --delete-generations` - delete profile generations
+
+# Synopsis
+
+`nix-env` `--delete-generations` *generations*
+
+# Description
+
+This operation deletes the specified generations of the current profile.
+
+*generations* can be a one of the following:
+
+- <span id="generations-list">[`<number>...`](#generations-list)</span>
+
+  A list of generation numbers, each one a separate command-line argument.
+
+  Delete exactly the profile generations given by their generation number.
+  Deleting the current generation is not allowed.
+
+- <span id="generations-old">[The special value `old`](#generations-old)</span>
+
+  Delete all generations except the current one.
+
+  > **WARNING**
+  >
+  > Older *and newer* generations will be deleted by this operation.
+  >
+  > One might expect this to just delete older generations than the curent one, but that is only true if the current generation is also the latest.
+  > Because one can roll back to a previous generation, it is possible to have generations newer than the current one.
+  > They will also be deleted.
+
+- <span id="generations-time">[`<number>d`](#generations-time)</span>
+
+  The last *number* days
+
+  *Example*: `30d`
+
+  Delete all generations created more than *number* days ago, except the most recent one of them.
+  This allows rolling back to generations that were available within the specified period.
+
+- <span id="generations-count">[`+<number>`](#generations-count)</span>
+
+  The last *number* generations up to the present
+
+  *Example*: `+5`
+
+  Keep the last *number* generations, along with any newer than current.
+
+Periodically deleting old generations is important to make garbage collection
+effective.
+The is because profiles are also garbage collection roots — any [store object] reachable from a profile is "alive" and ineligible for deletion.
+
+[store object]: @docroot@/store/store-object.md
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ./env-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+## Delete explicit generation numbers
+
+```console
+$ nix-env --delete-generations 3 4 8
+```
+
+Delete the generations numbered 3, 4, and 8, so long as the current active generation is not any of those.
+
+## Keep most-recent by count (number of generations)
+
+```console
+$ nix-env --delete-generations +5
+```
+
+Suppose `30` is the current generation, and we currently have generations numbered `20` through `32`.
+
+Then this command will delete generations `20` through `25` (`<= 30 - 5`),
+and keep generations `26` through `31` (`> 30 - 5`).
+
+## Keep most-recent by time (number of days)
+
+```console
+$ nix-env --delete-generations 30d
+```
+
+This command will delete all generations older than 30 days, except for the generation that was active 30 days ago (if it currently exists).
+
+## Delete all older
+
+```console
+$ nix-env --profile other_profile --delete-generations old
+```