README.md rewritten sentence by sentence (closes #219)

The old text served as the list of what a reader needs covered. The new
text leads with what quak is, what it is for and how to use it. Every
command, option, exit code and file layout follows in reference sections,
then the library reference, then design, entrypoints and development.
Every statement was checked against the code on next; the places where
the old text disagreed with it are listed in the pull request. No sentence
runs past about 30 words, every table is now a list, and no issue links or
project history remain in text meant for users.

Model: fable-5-1

src/cli-log.ts

@@ -1,18 +1,15 @@
 // The CLI's log level, and the one function the commands write stderr lines
 // through.
 //
-// Every line quak logs goes to stderr, at one of six levels, from the
+// Every line quak prints on stderr is a log line at one of six levels, from
-// quietest: fatal, error, warning, info, debug, trace. A run prints the lines
+// the quietest: fatal, error, warning, info, debug, trace. A run prints the
-// at its own level and at every quieter one. Its level is info unless `-v` or
+// lines at its own level and at every quieter one. Its level is info unless
-// `-q` asks for another (see "What quak prints" in the README). A command that
+// `-v` or `-q` asks for another (see "CLI surface" in the README). Every
-// runs and exits non-zero logs a fatal line saying why, so even `-q`, which
+// command that exits non-zero logs a fatal line saying why, so even `-q`, which
-// prints only fatal lines, never leaves such an exit unexplained. An error
+// prints only fatal lines, never leaves a non-zero exit unexplained. An error
 // line reports something that failed, such as a file or an album, and a
 // warning something that did not fail but needs the user's attention. stdout
-// carries a command's output and the prompts of `login`, and is not a log.
+// carries only a command's output and is not a log.
-// Two things on stderr are not log lines: the `error:` message for a
-// command line that quak cannot parse, and the usage printed when no command
-// is given.
 //
 // The library itself writes no lines: its layers report through callbacks, and
 // the CLI logs what they report. `logRequests` gives the API client options

test/cli/backup-end-to-end.test.ts

@@ -418,7 +418,7 @@ describe("the first backup of the account", () => {
         expect(downloads.sort()).toEqual([100, 200, 300]);
     });
 
-    // The tree under "The backup directory" in the README. Each original is at
+    // The tree "Backup layout" in the README describes. Each original is at
     // its save path, `YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID>.<ext>`, with
     // the extension of its title; the live photo is its image and its video,
     // each with the extension it had inside the ZIP, and a `.livephoto.json`

test/cli/log.test.ts

@@ -328,7 +328,7 @@ describe("what the library reports", () => {
         ]);
     });
 
-    // Each operation is named in plain words.
+    // Each operation is named in plain words, as the README names it.
     it("logs a refresh's progress at info, and its start and end at debug", () => {
         const { log, lines } = collect();
         const onProgress = logRefresh(log);