Add note about StreamField validation happening on the form #131

Job	Run time
test-sqlite (3.10, Django>=4.2,<4.3)	26s
test-postgres (3.8, Django>=4.2,<4.3, false, --parallel)	13s
test-postgres (3.11, Django>=5.0,<5.1, false, emailuser, --parallel)	14s
test-postgres (3.12, Django>=5.0,<5.1, postgres:15, notz, false, --parallel)	14s
test-mysql (3.9, Django>=4.2,<4.3, false)	15s
test-postgres (3.10, git+https://github.com/django/django.git@stable/5.0.x#egg=Django, true)	15s
test-postgres-elasticsearch7 (3.8, Django>=4.2,<4.3, false)	14s
test-mysql (3.12, Django>=5.0,<5.1, false, --parallel, mysql:8.1)	16s
test-postgres (3.10, git+https://github.com/django/django.git@main#egg=Django, true, postgres:lat...	14s
test-sqlite-elasticsearch8 (3.10, Django>=5.0,<5.1, emailuser)	23s
test-sqlite-opensearch2 (3.9, Django>=4.2,<4.3, false)	19s
coverage	1s
	3m 4s

Provide feedback