Fix documentation for git_repository and new_git_repository

Requiring either `build_file` or `build_file_content` is no longer true after
bazelbuild#16064

PiperOrigin-RevId: 517095299
Change-Id: Ifd32a50b633345048c1ee620d1883637ce85391f

tools/build_defs/repo/git.bzl

@@ -147,13 +147,11 @@ _common_attrs = {
  "This attribute is an absolute label (use '@//' for the main " +
  "repo). The file does not need to be named BUILD, but can " +
  "be (something like BUILD.new-repo-name may work well for " +
- "distinguishing it from the repository's actual BUILD files. " +
- "Either build_file or build_file_content must be specified.",
+ "distinguishing it from the repository's actual BUILD files. ",
  ),
  "build_file_content": attr.string(
  doc =
- "The content for the BUILD file for this repository. " +
- "Either build_file or build_file_content must be specified.",
+ "The content for the BUILD file for this repository. ",
  ),
  "workspace_file": attr.label(
  doc =