Recommend a `--prune` while `git fetch`ing

Especially with the main gitlab repository, there tends to be tons of
stray branches, a continue `git fetch` without any -p will easily lead
to hundreds of "orphaned branches" in the local checkouts.
Signed-off-by: Mattia Rizzolo <mattia@mapreri.org>

doc/update/10.5-to-10.6.md

@@ -103,7 +103,7 @@ rm go1.8.3.linux-amd64.tar.gz
 ```bash
 cd /home/git/gitlab
 
-sudo -u git -H git fetch --all
+sudo -u git -H git fetch --all --prune
 sudo -u git -H git checkout -- db/schema.rb # local changes will be restored automatically
 sudo -u git -H git checkout -- locale
 ```
@@ -131,7 +131,7 @@ sudo -u git -H git checkout 10-6-stable-ee
 ```bash
 cd /home/git/gitlab-shell
 
-sudo -u git -H git fetch --all --tags
+sudo -u git -H git fetch --all --tags --prune
 sudo -u git -H git checkout v$(</home/git/gitlab/GITLAB_SHELL_VERSION)
 sudo -u git -H bin/compile
 ```
@@ -146,7 +146,7 @@ If you are not using Linux you may have to run `gmake` instead of
 ```bash
 cd /home/git/gitlab-workhorse
 
-sudo -u git -H git fetch --all --tags
+sudo -u git -H git fetch --all --tags --prune
 sudo -u git -H git checkout v$(</home/git/gitlab/GITLAB_WORKHORSE_VERSION)
 sudo -u git -H make
 ```
@@ -182,7 +182,7 @@ sudo -u git -H sed -i.pre-10.1 's/\[\[storages\]\]/[[storage]]/' /home/git/gital
 
 ```shell
 cd /home/git/gitaly
-sudo -u git -H git fetch --all --tags
+sudo -u git -H git fetch --all --tags --prune
 sudo -u git -H git checkout v$(</home/git/gitlab/GITALY_SERVER_VERSION)
 sudo -u git -H make
 ```