Clarify kinit principal selection

Describe the principal selection behavior of kinit when the principal
argument is absent.

(cherry picked from commit 9896d4f)

ticket: 8403
version_fixed: 1.14.5

doc/user/user_commands/kinit.rst

@@ -33,7 +33,10 @@ DESCRIPTION
 -----------
 
 kinit obtains and caches an initial ticket-granting ticket for
-*principal*.
+*principal*.  If *principal* is absent, kinit chooses an appropriate
+principal name based on existing credential cache contents or the
+local username of the user invoking kinit.  Some options modify the
+choice of principal name.
 
 
 OPTIONS