sparse-checkout: improve docs around 'set' in cone mode

The existing documentation does not clarify how the 'set' subcommand
changes when core.sparseCheckoutCone is enabled. Correct this by
changing some language around the "A/B/C" example. Also include a
description of the input format matching the output of 'git ls-tree
--name-only'.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Documentation/git-sparse-checkout.txt

@@ -50,6 +50,14 @@ To avoid interfering with other worktrees, it first enables the
 +
 When the `--stdin` option is provided, the patterns are read from
 standard in as a newline-delimited list instead of from the arguments.
++
+When `core.sparseCheckoutCone` is enabled, the input list is considered a
+list of directories instead of sparse-checkout patterns. The command writes
+patterns to the sparse-checkout file to include all files contained in those
+directories (recursively) as well as files that are siblings of ancestor
+directories. The input format matches the output of `git ls-tree --name-only`.
+This includes interpreting pathnames that begin with a double quote (") as
+C-style quoted strings.
 
 'disable'::
 	Disable the `core.sparseCheckout` config setting, and restore the
@@ -128,9 +136,12 @@ the following patterns:
 ----------------
 
 This says "include everything in root, but nothing two levels below root."
-If we then add the folder `A/B/C` as a recursive pattern, the folders `A` and
-`A/B` are added as parent patterns. The resulting sparse-checkout file is
-now
+
+When in cone mode, the `git sparse-checkout set` subcommand takes a list of
+directories instead of a list of sparse-checkout patterns. In this mode,
+the command `git sparse-checkout set A/B/C` sets the directory `A/B/C` as
+a recursive pattern, the directories `A` and `A/B` are added as parent
+patterns. The resulting sparse-checkout file is now
 
 ----------------
 /*