Fix five README.md statements from the fifth review

Under `--json`, `snapshot list` prints no warning for a snapshot with
no manifest at the destination; the README now says so and points to
`remote_present`. The stdout list names `snapshot create` as well as
`snapshot restore` for the per-file error lines. `snapshot create`
stops "before it writes anything at the destination", since the
destination binding is saved first. `prune_grace_period` is a Go
duration, so the README now says only which units it rejects. The
commands that run while a backup is in progress are a bulleted list.

Model: fable-5-1

README.md

@@ -241,7 +241,7 @@ command that skipped anything still finishes, and then exits non-zero.
 `m` means minutes and `mo` means months. Days, weeks, months and years take
 whole numbers and can be combined, as in `2w3d`.
 
-`prune_grace_period` accepts only hours, minutes and seconds.
+`prune_grace_period` does not accept `d`, `w`, `mo` or `y`.
 
 ### `config init`
 
@@ -321,7 +321,7 @@ Flags:
 - `--keep-newer-than <duration>`: with `--prune`, keep every snapshot newer than
   this.
 
-The backup stops before it writes anything in these cases:
+The backup stops before it writes anything at the destination in these cases:
 
 - A `file://` destination directory does not exist.
 - The destination has no marker file, although the local index holds snapshots.
@@ -429,7 +429,9 @@ marker file although the local index holds snapshots.
 - `remote_present`: whether the snapshot is at the destination. It is `null`
   when the destination could not be listed.
 
-Under `--json`, every warning above goes to stderr.
+Under `--json`, every warning above goes to stderr, except the one about a
+snapshot with no manifest at the destination. That snapshot has
+`remote_present` set to `false`.
 
 ### `snapshot verify <snapshot-id>`
 
@@ -1074,8 +1076,14 @@ The lock covers one user on one machine. It does not stop another user or
 another machine from writing to the same destination. See
 [Sharing a destination](#sharing-a-destination).
 
-No other command takes the lock. `snapshot list`, `snapshot verify`,
+No other command takes the lock. These commands run while a backup is in
-`snapshot restore`, `info` and `remote info` run while a backup is in progress.
+progress:
+
+- `snapshot list`
+- `snapshot verify`
+- `snapshot restore`
+- `info`
+- `remote info`
 
 ## Output
 
@@ -1084,7 +1092,8 @@ No other command takes the lock. `snapshot list`, `snapshot verify`,
 stdout carries the output you asked for:
 
 - status and progress lines
-- warnings, and the errors that `snapshot restore` prints about files
+- warnings, and the errors that `snapshot create` and `snapshot restore` print
+  about files
 - tables and reports
 - JSON documents
 - `config get` values