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/opt-common.md

@@ -0,0 +1,38 @@
+# Options
+
+The following options are allowed for all `nix-env` operations, but may not always have an effect.
+
+- `--file` / `-f` *path*
+
+  Specifies the Nix expression (designated below as the *active Nix
+  expression*) used by the `--install`, `--upgrade`, and `--query
+  --available` operations to obtain derivations. The default is
+  `~/.nix-defexpr`.
+
+  If the argument starts with `http://` or `https://`, it is
+  interpreted as the URL of a tarball that will be downloaded and
+  unpacked to a temporary location. The tarball must include a single
+  top-level directory containing at least a file named `default.nix`.
+
+- `--profile` / `-p` *path*
+
+  Specifies the profile to be used by those operations that operate on
+  a profile (designated below as the *active profile*). A profile is a
+  sequence of user environments called *generations*, one of which is
+  the *current generation*.
+
+- `--dry-run`
+
+  For the `--install`, `--upgrade`, `--uninstall`,
+  `--switch-generation`, `--delete-generations` and `--rollback`
+  operations, this flag will cause `nix-env` to print what *would* be
+  done if this flag had not been specified, without actually doing it.
+
+  `--dry-run` also prints out which paths will be
+  [substituted](@docroot@/glossary.md) (i.e., downloaded) and which paths
+  will be built from source (because no substitute is available).
+
+- `--system-filter` *system*
+
+  By default, operations such as `--query --available` show derivations matching any platform. This option
+  allows you to use derivations for the specified platform *system*.