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

docs/ARCHITECTURE.md

@@ -173,7 +173,8 @@ A new `Scanner` is created for each snapshot, from `ScannerFactory`. It owns a
   had and that are now gone.
 - `batchAddFilesToSnapshot` records the unchanged files in the new snapshot.
 - `recordDirectories` records every directory, and the mount points found.
-- `processPhase` reads each changed file and stores it.
+- `processPhase` reads each changed regular file and stores it. It records each
+  changed symlink.
 
 The walk handles each kind of entry differently:
 

docs/DATAMODEL.md

@@ -316,7 +316,7 @@ in any of these:
 - link target
 
 A file is also read again when it lists a chunk that no uploaded blob holds. A
-file with no row in `files` is read too.
+regular file with no row in `files` is read too.
 
 Storing a changed file writes these rows: