Add explanatory note to the ZGW file import admin page

[swrichards]

This should eventually be better explained in the documentation, but for now it's good to have a temporary notice to guide the user.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.20%. Comparing base (18a524e) to head (a8841dc).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1367      +/-   ##
===========================================
+ Coverage    95.19%   95.20%   +0.01%     
===========================================
  Files         1003     1004       +1     
  Lines        37103    37173      +70     
===========================================
+ Hits         35321    35392      +71     
+ Misses        1782     1781       -1     

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[jiromaykin]

Hier en daar wat foutjes, die eigenlijk niets te maken met de codeverandering. Verder heb ik maar 1 suggestie voor een iets minder wollige tekst.

[alextreme]

Looks good!