Photo.savePath docs: a live photo not yet stored may move

For a live photo the backup has not stored, savePath carries the
title's extension, and the backup may store the image under the one
found inside the live photo. The comment on Photo.savePath and the
README now say so, as the comment on ContentCache.savePath does.

Model: opus-5-5

src/library/read.ts

@@ -89,9 +89,11 @@ export class Photo {
         return this.rec.isHidden;
     }
 
-    // The path `lib.backup()` writes the original to in the library's download
-    // directory, whether or not it is there yet; for a live photo, its image.
-    // Undefined when the library has no download directory or no content
+    // Where `lib.backup()` stores the original in the library's download
+    // directory, whether or not it is there yet: for a live photo already
+    // stored, its image. For a live photo not yet stored, it carries the
+    // title's extension, and the backup may store the image under a different
+    // one. Undefined when the library has no download directory or no content
     // cache.
     get savePath(): string | undefined {
         return this.cache?.savePath(this.rec.fileID);