Refine documentation for subpath

To remove ambiguity that the root would remain the same but only the subpath is preserved (i.e users could incorrectly think absolute paths within the repo would be preserved)

See related code:
https://github.com/carvel-dev/kapp-controller/blob/ad24bdc3e41219b176f88280a469e9fd21979339/pkg/fetch/vendir.go#L157-L157

Signed-off-by: Guillaume Berche <[email protected]>

Author: gberche-orange

pkg/apis/kappctrl/v1alpha1/types_fetch.go

@@ -100,7 +100,7 @@ type AppFetchGit struct {
 	// (if ssh-knownhosts is not specified, git will not perform strict host checking)
 	// +optional
 	SecretRef *AppFetchLocalRef `json:"secretRef,omitempty" protobuf:"bytes,3,opt,name=secretRef"`
-	// Grab only portion of repository (optional)
+	// Grab only portion of repository, subpath becomes new root (optional)
 	// +optional
 	SubPath string `json:"subPath,omitempty" protobuf:"bytes,4,opt,name=subPath"`
 	// Skip lfs download (optional)