Expanded upon directory exclusion for clarification

Sentence language seemed to contradict the section header, so it's
been clarified to prevent any confusion. Also added information
about how wikis are handled during a backup.

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6673.
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/raketasks/backup_restore.md

@@ -124,7 +124,7 @@ To use the `copy` strategy instead of the default streaming strategy, specify
 
 ### Excluding specific directories from the backup
 
-You can choose what should be backed up by adding the environment variable `SKIP`.
+You can choose what should be exempt from the backup up by adding the environment variable `SKIP`.
 The available options are:
 
 - `db` (database)
@@ -138,6 +138,9 @@ The available options are:
 
 Use a comma to specify several options at the same time:
 
+All wikis will be backed up as part of the `repositories` group. Non-existent wikis
+will be skipped during a backup.
+
 ```
 # use this command if you've installed GitLab with the Omnibus package
 sudo gitlab-rake gitlab:backup:create SKIP=db,uploads