Need to correct/improve the doc about Composer usage in each specific 5.x branch, since there are differences.
https://doc.ez.no/display/EZP/Installing+eZ+Publish+on+a+Linux-UNIX+based+system

Add a new section before "For pure legacy setup: Install Zetacomponents with Composer" to the tune of this:
5.3 and higher -> Install composer and run update to get latests versions of packages before you continue with installation. If you use a version control system, version composer.lock. And also link to composer page for info on how to keep system further up to date: https://doc.ez.no/display/EZP/Using+Composer
5.2: Same, but use composer for symfony and its dependencies only, not for eZ Publish and it's dependencies.

Then modify the section "For pure legacy setup: Install Zetacomponents with Composer": Main content as before, just make it clear that this applies to all 5.x branches. Also, if you're on 5.3 or 5.2 and installed Composer in the previous section, there's obviously no need to do it again.

The "Link assets" section should not be needed on 5.2 and 5.3 as it should be executed internally in previous steps, but we might need a note about it anyway if user needs to run in dev mode (QA needed on this).

Update: As mentioned in EZP-23342, Composer can be installed as executable, so it can be run simply as "composer ..." rather than "php -d memory_limit=-1 composer.phar ...". The long form will not work if it's set up this way, since composer.phar won't be found. Should this be Doc-ed? If so it must be added to all relevant docs, at least these:
https://doc.ez.no/display/EZP/Installing+eZ+Publish+on+a+Linux-UNIX+based+system
https://doc.ez.no/display/EZP/Using+Composer
https://github.com/ezsystems/ezpublish-platform/blob/stable-5.2/INSTALL.md