docs: document new description field

Also, explain GECOS syntax requirements.

1 files changed, 8 insertions, 6 deletions

diff --git a/docs/USER_RECORD.md b/docs/USER_RECORD.md
index f6d22c217b..960a82d894 100644
--- a/docs/USER_RECORD.md
+++ b/docs/USER_RECORD.md
@@ -221,12 +221,14 @@ optional, when unset the user should not be considered part of any realm. A
 user record with a realm set is never compatible (for the purpose of updates,
 see above) with a user record without one set, even if the `userName` field matches.
 
-`realName` → The real name of the user, a string. This should contain the user's
-real ("human") name, and corresponds loosely to the GECOS field of classic UNIX
-user records. When converting a `struct passwd` to a JSON user record this
-field is initialized from GECOS (i.e. the `pw_gecos` field), and vice versa
-when converting back. That said, unlike GECOS this field is supposed to contain
-only the real name and no other information.
+`realName` → The real name of the user, a string. This should contain the
+user's real ("human") name, and corresponds loosely to the GECOS field of
+classic UNIX user records. When converting a `struct passwd` to a JSON user
+record this field is initialized from GECOS (i.e. the `pw_gecos` field), and
+vice versa when converting back. That said, unlike GECOS this field is supposed
+to contain only the real name and no other information. This field must not
+contain control characters (such as `\n`) or colons (`:`), since those are used
+as record separators in classic `/etc/passwd` files and similar formats.
 
 `emailAddress` → The email address of the user, formatted as
 string. [`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html)