Say in --help where the database is and what each report prints (closes #177)

The help of sfdupes and of scan names the database, its default path
and SFDUPES_DATABASE. The help of report and trees lists the columns,
the order of the rows and the four escapes. The help of info lists its
lines. The help of dwim says that it scans /, should run as root and
keeps the scan in the database.

The README's opening drops its sentence on copies deleted after the
scan, an edge case that its report and trees sections already cover.

Model: opus-5-5

README.md

@@ -114,20 +114,15 @@ docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
 Run `scan` as root, so that it can read every file. `scan` warns about a
 directory it cannot read, and about a new or changed file it cannot read. It
 skips the path and deletes the records at or below it (see "Rules for the
-walk").
-
-An unmounted disk leaves its mount point empty, so `scan` deletes every record
-under it, and the next scan reads every file on the disk again. Test the mount
-point of each disk at or below `/srv` with its own `mountpoint -q`, joined with
-`&&`, and test `/srv` only when it is one. As the file `/etc/cron.d/sfdupes`:
+walk"). As the file `/etc/cron.d/sfdupes`:
 
 ```
-30 3 * * * root if mountpoint -q /srv; then /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log; else echo "a disk is not mounted; scan of /srv skipped"; fi
+30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
 ```
 
 - The first scan creates `/var/lib/sfdupes/db.sqlite` and its directory. To keep
-  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
-  before `/usr/local/bin/sfdupes`.
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` before the
+  command on the same line.
 - `scan` writes nothing to stdout. Its stderr, appended here to
   `/var/log/sfdupes.log`, holds:
     - one line naming the database;

TODO.md

@@ -29,9 +29,6 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
-- `README.md` says what a disk that is not mounted does to the database. The
-  daily `cron` line runs `scan` only when `mountpoint` reports `/srv` as mounted
-  (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/175)
 - The help of `sfdupes` and of `scan` names the database, its default path and
   `SFDUPES_DATABASE`. The help of `report` and `trees` lists the columns, the
   order of the rows and the escapes. The help of `info` lists its lines. The