docs: document the new proxy-related settings

The version of `github.com/prometheus/common` imported by Alertmanager
adds several fields to the `http_config` and `oauth2` structs for
setting proxy parameters:
* `no_proxy`
* `proxy_from_environment`
* `proxy_connect_header`

Signed-off-by: Simon Pasquier <[email protected]>

Author: simonpasquier

docs/configuration.md

@@ -557,6 +557,15 @@ oauth2:
 
 # Optional proxy URL.
 [ proxy_url: <string> ]
+# Comma-separated string that can contain IPs, CIDR notation, domain names
+# that should be excluded from proxying. IP and domain names can
+# contain port numbers.
+[ no_proxy: <string> ]
+# Use proxy URL indicated by environment variables (HTTP_PROXY, https_proxy, HTTPs_PROXY, https_proxy, and no_proxy)
+[ proxy_from_environment: <boolean> | default: false ]
+# Specifies headers to send to proxies during CONNECT requests.
+[ proxy_connect_header:
+  [ <string>: [<secret>, ...] ] ]
 
 # Configure whether HTTP requests follow HTTP 3xx redirects.
 [ follow_redirects: <bool> | default = true ]
@@ -597,6 +606,15 @@ tls_config:
 
 # Optional proxy URL.
 [ proxy_url: <string> ]
+# Comma-separated string that can contain IPs, CIDR notation, domain names
+# that should be excluded from proxying. IP and domain names can
+# contain port numbers.
+[ no_proxy: <string> ]
+# Use proxy URL indicated by environment variables (HTTP_PROXY, https_proxy, HTTPs_PROXY, https_proxy, and no_proxy)
+[ proxy_from_environment: <boolean> | default: false ]
+# Specifies headers to send to proxies during CONNECT requests.
+[ proxy_connect_header:
+  [ <string>: [<secret>, ...] ] ]
 ```
 
 #### `<tls_config>`