docs: cite SASL external authentication.

docs/FEATURES

@@ -134,8 +134,8 @@ SMB
  - authentication with NTLMv1
 
 SMTP
- - authentication: Plain, Login, CRAM-MD5, Digest-MD5, NTLM (*9) and Kerberos 5
-   (*4)
+ - authentication: Plain, Login, CRAM-MD5, Digest-MD5, NTLM (*9), Kerberos 5
+   (*4) and External.
  - send e-mails
  - mail from support
  - mail size support
@@ -150,8 +150,8 @@ SMTPS (*1)
 
 POP3
  - authentication: Clear Text, APOP and SASL
- - SASL based authentication: Plain, Login, CRAM-MD5, Digest-MD5, NTLM (*9) and
-   Kerberos 5 (*4)
+ - SASL based authentication: Plain, Login, CRAM-MD5, Digest-MD5, NTLM (*9),
+   Kerberos 5 (*4) and External.
  - list e-mails
  - retrieve e-mails
  - enhanced command support for: CAPA, DELE, TOP, STAT, UIDL and NOOP via
@@ -165,8 +165,8 @@ POP3S (*1)
 
 IMAP
  - authentication: Clear Text and SASL
- - SASL based authentication: Plain, Login, CRAM-MD5, Digest-MD5, NTLM (*9) and
-   Kerberos 5 (*4)
+ - SASL based authentication: Plain, Login, CRAM-MD5, Digest-MD5, NTLM (*9),
+   Kerberos 5 (*4) and External.
  - list the folders of a mailbox
  - select a mailbox with support for verifying the UIDVALIDITY
  - fetch e-mails with support for specifying the UID and SECTION

docs/TODO

@@ -494,7 +494,7 @@ Currently the SMB authentication uses NTLMv1.
 
 15.1 Other authentication mechanisms
 
- Add support for other authentication mechanisms such as EXTERNAL, OLP,
+ Add support for other authentication mechanisms such as OLP,
  GSS-SPNEGO and others.
  
 15.2 Add QOP support to GSSAPI authentication