Make more clear that building_area may not need building value to be filled

[matkoniecz]

Description, Motivation & Context

see #1857 (comment)

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[matkoniecz]

[matkoniecz]

@1ec5 review of English would be highly welcome

[matkoniecz]

@ZeLonewolf also pinging for review (thanks for all I got)

[ZeLonewolf]

Sounds fine to me