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/rollback.md

@@ -0,0 +1,34 @@
+# Name
+
+`nix-env --rollback` - set user environment to previous generation
+
+# Synopsis
+
+`nix-env` `--rollback`
+
+# Description
+
+This operation switches to the “previous” generation of the active
+profile, that is, the highest numbered generation lower than the current
+generation, if it exists. It is just a convenience wrapper around
+`--list-generations` and `--switch-generation`.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ./env-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+```console
+$ nix-env --rollback
+switching from generation 92 to 91
+```
+
+```console
+$ nix-env --rollback
+error: no generation older than the current (91) exists
+```