[PATCH pve-docs 1/1] notifications: Add section about OAuth2 to SMTP targets docs

[Arthur Bied-Charreton <a.bied-charreton@proxmox.com>]

Document the new config entries, and add notes/warnings to communicate
that:

1. User intervention is required for initial OAuth2 target setup, and

2. Microsoft OAuth2 apps *must not* be configured as SPAs by the
user, since it would prevent PVE from automatically extending the
refresh token's lifetime

Signed-off-by: Arthur Bied-Charreton <a.bied-charreton@proxmox.com>
---
 notifications.adoc | 44 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/notifications.adoc b/notifications.adoc
index 801b327..679b19b 100644
--- a/notifications.adoc
+++ b/notifications.adoc
@@ -108,16 +108,23 @@ The configuration for SMTP target plugins has the following options:
 * `from-address`: Sets the From-address of the email. SMTP relays might require
   that this address is owned by the user in order to avoid spoofing.  The `From`
   header in the email will be set to `$author <$from-address>`.
+* `auth-method`: Sets the authentication method (`plain`, `google-oauth2` or
+  `microsoft-oauth2`).
 * `username`: Username to use during authentication. If no username is set,
   no authentication will be performed. The PLAIN and LOGIN authentication
   methods are supported.
 * `password`: Password to use when authenticating.
+* `oauth2-client-id`: Client ID for the OAuth2 application, if applicable.
+* `oauth2-client-secret`: Client secret for the OAuth2 application, if
+  applicable.
+* `oauth2-tenant-id`: Tenant ID for the OAuth2 application, if applicable.
+  Only required for Microsoft OAuth2.
 * `mode`: Sets the encryption mode (`insecure`, `starttls` or `tls`). Defaults
   to `tls`.
 * `server`: Address/IP of the SMTP relay.
-* `port`: The port to connect to. If not set, the used port .
-   Defaults to 25 (`insecure`), 465 (`tls`) or 587 (`starttls`), depending on
-   the value of `mode`.
+* `port`: The port to connect to. If not set, the used port defaults to 25
+  (`insecure`), 465 (`tls`) or 587 (`starttls`), depending on the value of
+  `mode`.
 * `comment`: Comment for this target
 
 Example configuration (`/etc/pve/notifications.cfg`):
@@ -133,13 +140,42 @@ smtp: example
 ----
 
 The matching entry in `/etc/pve/priv/notifications.cfg`, containing the
-secret token:
+password:
 
 ----
 smtp: example
         password somepassword
 ----
 
+[[notification_targets_smtp_oauth2]]
+===== OAuth2 Authentication
+
+SMTP targets also support OAuth2 authentication via the XOAUTH2 mechanism for
+Google and Microsoft mail providers.
+
+Setting up OAuth2 authentication requires creating an OAuth2 application with
+the chosen provider. The application must be configured with a redirect URI
+pointing to the {pve} web interface, i.e. the URL from which the initial
+authentication request is performed in the UI.
+
+CAUTION: For Microsoft, the application must *not* be registered as Single-Page
+Application (SPA), as the lifetime of refresh tokens granted for SPAs cannot
+be extended automatically by {pve}.
+
+To set up OAuth2 authentication via the web interface, select `OAuth2 (Google)`
+or `OAuth2 (Microsoft)` as the authentication method, fill in the client ID and
+secret (and the tenant ID for Microsoft), then click the *Authenticate* button.
+This opens a new window where you can sign in with the selected provider and
+grant the required permissions. Upon successful authentication, a refresh
+token is obtained and stored automatically.
+
+Token refresh happens automatically, manual intervention is only needed if a
+token is revoked.
+
+NOTE: OAuth2 is currently not configurable through direct configuration file
+editing because the refresh token is managed as dynamic state by {pve}. All
+OAuth2 targets must be configured via the web interface.
+
 [[notification_targets_gotify]]
 Gotify
 ~~~~~~
-- 
2.47.3