docs: Fix description of `DecimalField`'s `max_digits` (#4714)
docs: Fix description of `DecimalField`'s `max_digits` (#4714)

As of PR #4377, `max_digits=None` is allowed for `DecimalField`.
tomchristie/django-rest-framework,atombrella/django-rest-framework,atombrella/django-rest-framework,atombrella/django-rest-framework,jpadilla/django-rest-framework,jpadilla/django-rest-framework,kgeorgy/django-rest-framework,edx/django-rest-framework,ossanna16/django-rest-framework,davesque/django-rest-framework,linovia/django-rest-framework,jpadilla/django-rest-framework,ossanna16/django-rest-framework,uploadcare/django-rest-framework,uploadcare/django-rest-framework,edx/django-rest-framework,tomchristie/django-rest-framework,dmwyatt/django-rest-framework,ossanna16/django-rest-framework,linovia/django-rest-framework,kgeorgy/django-rest-framework,linovia/django-rest-framework,dmwyatt/django-rest-framework,uploadcare/django-rest-framework,edx/django-rest-framework,davesque/django-rest-framework,davesque/django-rest-framework,dmwyatt/django-rest-framework,kgeorgy/django-rest-framework,tomchristie/django-rest-framework