manifest-format: document implicit directory creation w/<copyfile> & <linkfile>

Bug: https://crbug.com/gerrit/11218
Change-Id: Ie96b4c484d9fbfd550c580c3d02971dc088dd8b0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/233052
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Jonathan Nieder <jrn@google.com>

docs/manifest-format.md

@@ -330,6 +330,8 @@ client is not allowed.
 "src" and "dest" must be files.  Directories or symlinks are not allowed.
 Intermediate paths must not be symlinks either.
 
+Parent directories of "dest" will be automatically created if missing.
+
 ### Element linkfile
 
 It's just like copyfile and runs at the same time as copyfile but
@@ -338,6 +340,8 @@ instead of copying it creates a symlink.
 The symlink is created at "dest" (relative to the top of the tree) and
 points to the path specified by "src".
 
+Parent directories of "dest" will be automatically created if missing.
+
 The symlink target may be a file or directory, but it may not point outside
 of the repo client.