Add explanatory comments for weird char field length limits

dandiapi/api/models/audit.py

@@ -33,8 +33,20 @@
 class AuditRecord(models.Model):
     timestamp = models.DateTimeField(auto_now_add=True)
     dandiset_id = models.IntegerField()
+
+    # GitHub enforces a 39 character limit on usernames (see, e.g.,
+    # https://docs.github.com/en/enterprise-cloud@latest/admin/managing-iam/iam-configuration-reference/username-considerations-for-external-authentication).
     username = models.CharField(max_length=39)
+
+    # According to RFC 5321 (https://www.rfc-editor.org/rfc/rfc5321.txt),
+    # section 4.5.3.1.3, an email address "path" is limited to 256 octets,
+    # including the surrounding angle brackets. Without the brackets, that
+    # leaves 254 characters for the email address itself.
     user_email = models.CharField(max_length=254)
+
+    # The signup questionnaire imposes a 150 character limit on both first and
+    # last names; together with a space to separate them, that makes a 301
+    # character limit on the full name.
     user_fullname = models.CharField(max_length=301)
     record_type = models.CharField(max_length=32, choices=AUDIT_RECORD_CHOICES)
     details = models.JSONField(blank=True)