docs(repo): make explicit which branch is used when creating a repo

This adds a line of documentation in the `gh repo create` command's help
specifying which branch for the new repo is selected.

pkg/cmd/repo/create/create.go

@@ -99,6 +99,8 @@ func NewCmdCreate(f *cmdutil.Factory, runF func(*CreateOptions) error) *cobra.Co
 			For language or platform .gitignore templates to use with %[1]s--gitignore%[1]s, <https://github.com/github/gitignore>.
 
 			For license keywords to use with %[1]s--license%[1]s, run %[1]sgh repo license list%[1]s or visit <https://choosealicense.com>.
+
+			The repo is created with the configured repository default branch, see <https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/changing-the-default-branch>.
 		`, "`"),
 		Example: heredoc.Doc(`
 			# create a repository interactively