Say that a symlink is recorded, not read

A new or changed symlink goes through recordNonRegularFile in the
scanner and is never opened. README.md, ARCHITECTURE.md and
docs/DATAMODEL.md now say so where they describe what a backup reads,
and README.md no longer lists a changed link target as a reason to
read a file.

Model: fable-5-1

README.md

@@ -326,11 +326,11 @@ holds:
 - Its modification time has changed.
 - Its mode has changed.
 - Its owner or group has changed.
-- Its link target has changed.
 - No uploaded blob holds one of its chunks. This happens when the only snapshot
   that used the blob holding that chunk is removed.
 
-Every other file is recorded in the new snapshot without being read.
+Every other file is recorded in the new snapshot without being read. A symlink
+has no content to read, so it is only recorded.
 
 Flags:
 
@@ -730,8 +730,8 @@ Deletes the local index file. The destination is not changed.
 It needs a config file that loads, because the config gives the path of the
 index.
 
-The next backup then reads every file again. It skips the upload of a blob that
-the destination already holds under the same name.
+The next backup then reads every regular file again. It skips the upload of a
+blob that the destination already holds under the same name.
 
 Use this command after you point the config at a different destination. See
 [Changing the destination](#changing-the-destination).