Amend user docs per code repo PR #12037

[tomeg09]

Note: pending approval of PR to jabref code repository: JabRef/jabref#12037

This PR amends the user docs to reflect changes in Jabref's General File Directory to User-Specific File Directory.

@koppor: the following line has been excluded as it appears to refer to a different functionality. please let me know if my understanding is incorrect:

{% hint style="info" %}
The legacy PDF/PS links (i.e. the pdf and ps fields, which were used in JabRef versions prior to 2.3), should in current versions be replaced by general file links. This can be done using **Quality → Cleanup entries...** and enabling _Upgrade external PDF/PS links to use the 'file' field_.​
{% endhint %}

[koppor]

@koppor: the following line has been excluded

Thank you for thinking about it! You are right!