Using Django docs to find widget readonly specification

I'm new to learning Django.  I want to find the documentation regarding the readonly attribute you can set for admin widgets.  For example, from an admin.py:

class SourceForm(forms.ModelForm):

  data = forms.CharField(widget=TextInput(attrs={'readonly': True, 'class': 'averageField'}))

  class Meta:

    model = Source

class SourceAdmin(admin.ModelAdmin):

  form = SourceForm

admin.site.register(Source, SourceAdmin)

I've search for readonly on https://docs.djangoproject.com/search/?q=readonly&release=7 but did not found anything.  Also searching manually in https://docs.djangoproject.com/en/dev/ref/forms/widgets/ didn't bring anything up.  Can someone tell me where I'm going wrong please.

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