Merge branch '41607-require-rsync-docs' into 'master'

Resolve "Document rsync as requirement for 10.4"

Closes #41607

See merge request gitlab-org/gitlab-ce!16252

doc/install/installation.md

@@ -64,7 +64,7 @@ up-to-date and install it.
 
 Install the required packages (needed to compile Ruby and native extensions to Ruby gems):
 
-    sudo apt-get install -y build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libre2-dev libreadline-dev libncurses5-dev libffi-dev curl openssh-server checkinstall libxml2-dev libxslt-dev libcurl4-openssl-dev libicu-dev logrotate python-docutils pkg-config cmake
+    sudo apt-get install -y build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libre2-dev libreadline-dev libncurses5-dev libffi-dev curl openssh-server checkinstall libxml2-dev libxslt-dev libcurl4-openssl-dev libicu-dev logrotate rsync python-docutils pkg-config cmake
 
 Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but
 you can [install re2 manually](https://github.com/google/re2/wiki/Install).

doc/raketasks/backup_restore.md

@@ -14,6 +14,19 @@ from one server to another is through backup restore.
 GitLab provides a simple command line interface to backup your whole installation,
 and is flexible enough to fit your needs.
 
+### Requirements
+
+If you're using GitLab with the Omnibus package, you're all set. If you
+installed GitLab from source, make sure the following packages are installed:
+
+* rsync
+
+If you're using Ubuntu, you could run:
+
+```
+sudo apt-get install -y rsync
+```
+
 ### Backup timestamp
 
 >**Note:**

doc/update/10.3-to-10.4.md

@@ -21,6 +21,8 @@ sudo service gitlab stop
 
 ### 2. Backup
 
+NOTE: If you installed GitLab from source, make sure `rsync` is installed.
+
 ```bash
 cd /home/git/gitlab