docs: document internal manifests.git/config settings

Change-Id: I6b32d925756375a9335522ff33376cb5f7ed1157
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254073
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

docs/internal-fs-layout.md

@@ -103,6 +103,29 @@ support, see the [manifest-format.md] file.
 *   `subprojects/`: Like `projects/`, but for git submodules.
 *   `subproject-objects/`: Like `project-objects/`, but for git submodules.
 
+### Settings
+
+The `.repo/manifests.git/config` file is used to track settings for the entire
+repo client checkout.
+Most settings use the `[repo]` section to avoid conflicts with git.
+User controlled settings are initialized when running `repo init`.
+
+| Setting           | `repo init` Option        | Use/Meaning |
+|-------------------|---------------------------|-------------|
+| manifest.groups   | `--groups` & `--platform` | The manifest groups to sync |
+| repo.archive      | `--archive`               | Use `git archive` for checkouts |
+| repo.clonefilter  | `--clone-filter`          | Filter setting when using [partial git clones] |
+| repo.depth        | `--depth`                 | Create shallow checkouts when cloning |
+| repo.dissociate   | `--dissociate`            | Dissociate from any reference/mirrors after initial clone |
+| repo.mirror       | `--mirror`                | Checkout is a repo mirror |
+| repo.partialclone | `--partial-clone`         | Create [partial git clones] |
+| repo.reference    | `--reference`             | Reference repo client checkout |
+| repo.submodules   | `--submodules`            | Sync git submodules |
+| user.email        | `--config-name`           | User's e-mail address; Copied into `.git/config` when checking out a new project |
+| user.name         | `--config-name`           | User's name; Copied into `.git/config` when checking out a new project |
+
+[partial git clones]: https://git-scm.com/docs/gitrepository-layout#_code_partialclone_code
+
 ## ~/ dotconfig layout
 
 Repo will create & maintain a few files in the user's home directory.