Update doc of AshPhoenix.Form.add_error/3 about visibility #1078

Job	Run time
ash-ci / audit	52s
ash-ci / mix deps.get	24s
ash-ci / mix igniter.upgrade	4s
ash-ci / MIX_ENV=test mix.compile	38s
ash-ci / mix format --check-formatted	39s
ash-ci / mix spark.cheat_sheets --check	25s
ash-ci / mix spark.formatter --check	4s
ash-ci / MIX_ENV=dev mix.compile	32s
ash-ci / mix docs	26s
ash-ci / mix credo --strict	3s
ash-ci / mix dialyzer	3m 45s
ash-ci / mix sobelow	25s
ash-ci / mix test	55s
ash-ci / mix deps.unlock --check-unused	22s
ash-ci / mix ash.codegen --check	0s
ash-ci / Deploy docs to GitHub pages.	0s
ash-ci / Release	0s
	9m 34s

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