[QA] improve imagick update instructions

[jnweiger]

WHAT Needs to be Documented?

The documentation is complete, but here are a few points that should be reviewed/updated:

• Secure ImageMagick section seems obsolete. The policy.xml files of 6 or 7 do not contain pattern="MSL"
• echo (with no parameters) is a simpler alternative to printf "\n" -- I'd suggest to use echo.
• Check if ImageMagic-7 and its libraries have been properly installed dpkg -l | grep imei - this command reports nothing, although I have a working imagic7 after running imei.sh - What should it report?
• sudo pecl channel-update pecl.php.net requires that php-pear is installed. That should be mentioned.
• After installing ImageMagick-7 and if you do not define the configuration location, the default configuration files can be found at /usr/local/etc/ImageMagick-7
  -> not in my case. /usr/local/etc/ is still empty after imei.sh was run.

WHERE Does This Need To Be Documented (Link)?

https://github.com/owncloud/docs-server/edit/master/modules/admin_manual/pages/installation/manual_installation/manual_imagick7.adoc

WHY Should This Change Be Made?

Several things don't work for me as described. The Original author of the text should review this.

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other