Rewrite README.md sentence by sentence for a newcomer

The old README was one dense specification: long sentences, lists
chained with commas, exceptions ahead of the rules, and release
history in the user text. This rewrite leads with what the program is
and how to use it, then gives a reference for the settings, each
subcommand, the database, the output formats, the progress display
and the exit codes. Non-goals come before the reference; the
constraints, the entrypoints and the build come last. Every statement
was checked against the code on next and against the built binary on
small trees. The section names the code comments cite are kept. Five
comments in main_test.go cite Settings, and one each in scan_test.go,
trees_test.go and exclude_test.go cites what the README now says.
TODO.md records the step.

Model: fable-5-1

README.md

@@ -365,7 +365,7 @@ default. An operand may be a directory or a regular file.
 - No other case keeps an operand under another. An operand reached through a
   symlinked directory below the outer operand is dropped too, without a line.
   The outer operand's walk does not follow the symlink, so no file under the
-  inner operand is scanned. The records that an earlier scan left under it are
+  inner operand is scanned. The records an earlier scan left under it are
   deleted.
 - An operand that is neither a regular file nor a directory is not scanned.
   `scan` prints a warning naming the path and what it is. It counts the operand
@@ -466,8 +466,8 @@ walk practical on large or busy storage pools.
     operand at the path given, even when that path runs through a symlinked
     directory. Such an operand can lead to the database files, and the walk
     passes over them there as well. The walk prints no line for the database
-    files and does not count them as skipped. A record that an earlier scan left
-    for one of them is deleted.
+    files and does not count them as skipped. A record an earlier scan left for
+    one of them is deleted.
 
 - By default the walk enters directories on other filesystems than its
   operand's. With `-x`, it never enters a directory on a different filesystem
@@ -556,8 +556,7 @@ walk practical on large or busy storage pools.
 - A file or directory that disappears while the scan runs is not an error. The
   scan prints no line for it and does not count it as skipped. A path gone when
   the walk reaches it gets no record, and so does a file gone when the hash
-  phase opens it. The records that an earlier scan left at or below it are
-  deleted.
+  phase opens it. The records an earlier scan left at or below it are deleted.
 
 #### Concurrency
 
@@ -669,8 +668,8 @@ Four bytes are written as escapes:
 
 Every other byte is written unchanged, including bytes that are not valid UTF-8,
 the usual text encoding. Undoing the four escapes gives back the stored path.
-Grouping and ordering use the stored path. The warnings about skipped paths that
-`scan` prints are escaped the same way, so each stays one line.
+Grouping and ordering use the stored path. The warnings that `scan` prints are
+escaped the same way, so each warning stays one line.
 
 The summary line on stderr gives:
 
@@ -1038,9 +1037,9 @@ On a terminal:
     - the elapsed time;
     - the time remaining.
 
-    On a terminal 80 columns wide, no line is cut short, even hours into a scan
-    of millions of files. A terminal made narrower during a phase is fitted at
-    the next draw.
+    On a terminal narrower than about 75 columns, a line is cut short only hours
+    into a scan of millions of files. A terminal made narrower during a phase is
+    fitted at the next draw.
 
 - A warning printed during a phase lands on a line of its own, never inside the
   display. So does every line written to stdout when stdout and stderr are the

main_test.go

@@ -773,8 +773,8 @@ func TestBuildVersion(t *testing.T) {
 func TestRunVersionWriteFailureIsFatal(t *testing.T) {
 	t.Parallel()
 
-	// README §Error handling: a failed write to stdout is a fatal error,
-	// exit 1, reported on stderr starting with "sfdupes:".
+	// README §Error handling: a stdout write failure exits 1, reported
+	// in one line on stderr.
 	var stderr bytes.Buffer
 
 	code := run([]string{"--version"}, failingWriter{}, &stderr)
@@ -1445,8 +1445,8 @@ func TestRunReportsDuringScan(t *testing.T) {
 }
 
 func TestRunStdoutClosedIsFatal(t *testing.T) {
-	// README §Error handling: a failed write to stdout is a fatal error,
-	// exit 1, reported on stderr starting with "sfdupes:".
+	// README §Error handling: a stdout write failure exits 1, reported
+	// in one line on stderr.
 	for _, name := range []string{cmdReport, cmdTrees, cmdInfo} {
 		t.Run(name, func(t *testing.T) {
 			t.Setenv(databaseEnv, testDBPath(t))