cron,services/index: Multiple changes.

* cron:
  - link to "cron" page on Wikipedia.
  - provide brief introduction.
  - mention crontab(1).
  - mention the `crond` service.
  - remove reference to `runwhen`, which is not packaged.
  - expand description of `snooze` package.
* services/index:
  - mention option of using `cron` to run something regularly, link to
  "cron" section.

Author: flexibeast

src/config/cron.md

@@ -1,13 +1,27 @@
 # Cron
 
-Void Linux comes without a default cron daemon, you can choose one of multiple
-cron implementations, by installing the package and enabling the system service.
+[cron](https://en.wikipedia.org/wiki/Cron) is a daemon for running programs at
+regular intervals. The programs and intervals are specified in a `crontab` file,
+which can be edited with [crontab(1)](https://man.voidlinux.org/crontab.1).
+Running `crontab -e` as the superuser will edit the system crontab; otherwise,
+it will edit the crontab for the current user.
 
-Available choices include [cronie](https://github.com/cronie-crond/cronie/),
+By default, a cron daemon is not installed. However, multiple cron
+implementations are available, including
+[cronie](https://github.com/cronie-crond/cronie/),
 [dcron](http://www.jimpryor.net/linux/dcron.html),
 [fcron](http://fcron.free.fr/) and more.
 
-As alternative to the standard cron implementations you can use something like
-[snooze](https://github.com/leahneukirchen/snooze) or
-[runwhen](https://code.dogmap.org/runwhen/) which go hand in hand with service
-supervision.
+Once you have chosen and installed an implementation,
+[enable](./services/index.md#enabling-services) the `crond` service, which is a
+symlink to the actual service (e.g. `dcron`). If you install several
+implementations, you can choose which one to use via
+[xbps-alternatives(1)](https://man.voidlinux.org/xbps-alternatives.1); this will
+alter the `crond` symlink appropriately. Implementation documentation will be
+available in [crond(8)](https://man.voidlinux.org/crond.8).
+
+As an alternative to the standard cron implementations, you can use
+[snooze(1)](https://man.voidlinux.org/snooze.1) together with the
+`snooze-hourly`, `snooze-daily`, `snooze-weekly` and `snooze-monthly` services,
+which are provided by the `snooze` package for this purpose. Each of these
+services execute scripts in the respective `/etc/cron.*` directories.

src/config/services/index.md

@@ -12,6 +12,10 @@ Some advantages of using runit include:
 - a reliable logging facility for services, where the log service stays up as
    long as the relevant service is running and possibly writing to the log.
 
+If you don't need a program to be running constantly, but would like it to run
+at regular intervals, you might like to consider using a [cron
+daemon](../cron.md).
+
 ## Section Contents
 
 - [Per-User Services](./user-services.md)