Add "log_file" command hook context to NEWS and docs (#413).

NEWS

@@ -1,4 +1,6 @@
 1.7.12.dev0
+ * #413: Add "log_file" context to command hooks. See the documentation for more information:
+   https://torsion.org/borgmatic/docs/how-to/add-preparation-and-cleanup-steps-to-backups/
  * #666: Fix error when running the "info" action with the "--match-archives" flag. Also fix the
    "--match-archives" flag to correctly override the "match_archives" configuration option for
    the "transfer", "list", "rlist", and "info" actions.

docs/how-to/add-preparation-and-cleanup-steps-to-backups.md

@@ -66,6 +66,9 @@ variables you can use here:
 
  * `configuration_filename`: borgmatic configuration filename in which the
    hook was defined
+ * `log_file`
+   <span class="minilink minilink-addedin">New in version 1.7.12</span>:
+   path of the borgmatic log file, only set when the `--log-file` flag is used
  * `repository`: path of the current repository as configured in the current
    borgmatic configuration file