doc/fetchers: document leaveDotGit, deepClone and fetchSubmodules

doc/builders/fetchers.chapter.md

@@ -31,6 +31,8 @@ Used with Subversion. Expects `url` to a Subversion directory, `rev`, and `sha25
 
 Used with Git. Expects `url` to a Git repo, `rev`, and `sha256`. `rev` in this case can be full the git commit id (SHA1 hash) or a tag name like `refs/tags/v1.0`.
 
+Additionally the following optional arguments can be given: `fetchSubmodules = true` makes `fetchgit` also fetch the submodules of a repository. If `deepClone` is set to true, the entire repository is cloned as opposing to just creating a shallow clone. `deepClone = true` also implies `leaveDotGit = true` which means that the `.git` directory of the clone won't be removed after checkout.
+
 ## `fetchfossil`
 
 Used with Fossil. Expects `url` to a Fossil archive, `rev`, and `sha256`.
@@ -49,6 +51,8 @@ A number of fetcher functions wrap part of `fetchurl` and `fetchzip`. They are m
 
 `fetchFromGitHub` expects four arguments. `owner` is a string corresponding to the GitHub user or organization that controls this repository. `repo` corresponds to the name of the software repository. These are located at the top of every GitHub HTML page as `owner`/`repo`. `rev` corresponds to the Git commit hash or tag (e.g `v1.0`) that will be downloaded from Git. Finally, `sha256` corresponds to the hash of the extracted directory. Again, other hash algorithms are also available but `sha256` is currently preferred.
 
+`fetchFromGitHub` uses `fetchzip` to download the source archive generated by GitHub for the specified revision. If `leaveDotGit`, `deepClone` or `fetchSubmodules` are set to `true`, `fetchFromGitHub` will use `fetchgit` instead. Refer to its section for documentation of these options.
+
 ## `fetchFromGitLab`
 
 This is used with GitLab repositories. The arguments expected are very similar to fetchFromGitHub above.