ITS#9839 Add note for ldap_url_parse() with port 0

In OpenLDAP 2.6.x and earlier, when the port is set to 0 in url string
explicitly, ldap_url_parse() will set lud_port of struct ldap_url_desc
to the default port. This behavior is not documented.
Signed-off-by: Jiaqing Zhao <jiaqing.zhao@linux.intel.com>

doc/man/man3/ldap_url.3

@@ -57,6 +57,9 @@ URLs that are wrapped in angle-brackets and/or preceded by "URL:" are also
 tolerated.  Alternative LDAP schemes such as ldaps:// and ldapi:// may be
 parsed using the below routines as well.
 .LP
+If the optional \fIportnumber\fP in URL is 0, default port (389 for LDAP,
+636 for LDAPS) will be assigned to \fIlud_port\fP.
+.LP
 .B ldap_is_ldap_url()
 returns a non-zero value if \fIurl\fP looks like an LDAP URL (as
 opposed to some other kind of URL).  It can be used as a quick check