docs(Textarea): add notice about form

[layershifter]

Refs #3242.
Replaces #3243.

This adds a note about required markup.

[codecov-io]

Codecov Report

Merging #3269 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3269   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         169      169           
  Lines        2803     2803           
=======================================
  Hits         2801     2801           
  Misses          2        2

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