[needs-docs] Rename simplify_vw to simplify_by_area

[DelazJ]

fixes #16407
The idea is to have a more meaningful name than before... @nyalldawson ?

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[nyalldawson]

We have a loose convention that expression functions follow the same name as their equivalent postgres/postgis, but dropping any "st_" prefixes. In this case the equivalent postgis function is "st_simplifyvw".

I do like your name better, but I'm reluctant to vary from this convention.

[DelazJ]

I was not sure if we were still following the PG functions naming. And if we do, yes, let's keep the equivalent naming although it's not obvious. 😢
Thanks