doc: sparse-checkout: mention --cone option

In af09ce2 ("sparse-checkout: init and set in cone mode", 2019-11-21),
the '--cone' option was added to 'git sparse-checkout init'.

Document it.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-sparse-checkout.txt

@@ -41,6 +41,10 @@ COMMANDS
 To avoid interfering with other worktrees, it first enables the
 `extensions.worktreeConfig` setting and makes sure to set the
 `core.sparseCheckout` setting in the worktree-specific config file.
++
+When `--cone` is provided, the `core.sparseCheckoutCone` setting is
+also set, allowing for better performance with a limited set of
+patterns (see 'CONE PATTERN SET' below).
 
 'set'::
 	Write a set of patterns to the sparse-checkout file, as given as