remove indentation in fetchGit attribute listing

also reword a confusing sentence and add links to Git terminology

src/libexpr/primops/fetchTree.cc

@@ -353,36 +353,44 @@ static RegisterPrimOp primop_fetchGit({
       of the repo at that URL is fetched. Otherwise, it can be an
       attribute with the following attributes (all except `url` optional):
 
-        - url\
+      - `url`
+
         The URL of the repo.
 
-        - name\
+      - `name` (default: *basename of the URL*)
-          The name of the directory the repo should be exported to in the
-          store. Defaults to the basename of the URL.
 
-        - rev\
+        The name of the directory the repo should be exported to in the store.
-          The git revision to fetch. Defaults to the tip of `ref`.
 
-        - ref\
+      - `rev` (default: *the tip of `ref`*)
-          The git ref to look for the requested revision under. This is
-          often a branch or tag name. Defaults to `HEAD`.
 
-          By default, the `ref` value is prefixed with `refs/heads/`. As
+        The [Git revision] to fetch.
-          of Nix 2.3.0 Nix will not prefix `refs/heads/` if `ref` starts
+        This is typically a commit hash.
-          with `refs/`.
 
-        - submodules\
+        [Git revision]: https://git-scm.com/docs/git-rev-parse#_specifying_revisions
-          A Boolean parameter that specifies whether submodules should be
-          checked out. Defaults to `false`.
 
-        - shallow\
+      - `ref` (default: `HEAD`)
-          A Boolean parameter that specifies whether fetching a shallow clone
-          is allowed. Defaults to `false`.
 
-        - allRefs\
+        The [Git reference] under which to look for the requested revision.
-          Whether to fetch all refs of the repository. With this argument being
+        This is often a branch or tag name.
-          true, it's possible to load a `rev` from *any* `ref` (by default only
+
-          `rev`s from the specified `ref` are supported).
+        [Git reference]: https://git-scm.com/book/en/v2/Git-Internals-Git-References
+
+        By default, the `ref` value is prefixed with `refs/heads/`.
+        As of 2.3.0, Nix will not prefix `refs/heads/` if `ref` starts with `refs/`.
+
+      - `submodules` (default: `false`)
+
+        A Boolean parameter that specifies whether submodules should be checked out.
+
+      - `shallow` (default: `false`)
+
+        A Boolean parameter that specifies whether fetching a shallow clone is allowed.
+
+      - `allRefs`
+
+        Whether to fetch all references of the repository.
+        With this argument being true, it's possible to load a `rev` from *any* `ref`
+        (by default only `rev`s from the specified `ref` are supported).
 
       Here are some examples of how to use `fetchGit`.