update blurb for ldap.accountFullName

Specifying ldap.accountFullName is optional, but if it is not set,
then it gets a default value and that default value prevents edits to
"Full Name" inside Gerrit.  To allow edits to "Full Name" (which is
useful when ldap account data contains typos), you need to explicitly
set this to an empty string.  Reword the blurb to explain this.

Release-Notes: Improved documentation
Change-Id: I7a4c8f09ee2b016ec9b26e8fb8faefe162830023
diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 9d347e8..2e175e6 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -3924,11 +3924,12 @@
 example to join given name and surname together, use the pattern
 `${givenName} ${SN}`.
 +
-If set, users will be unable to modify their full name field, as
-Gerrit will populate it only from the LDAP data.
-+
 Default is `displayName` for FreeIPA and RFC 2307 servers,
 and `${givenName} ${sn}` for Active Directory.
++
+A non-empty or default value prevents users from modifying their full
+name field.  To allow edits to the full name field, set to the empty
+string.
 
 [[ldap.accountEmailAddress]]ldap.accountEmailAddress::
 +