Update resolve_timeout docs to indicate it is a fallback value (prometheus#1492)

* Update resolve_timeout docs to indicate it is a fallback value
* Update resolve_timeout documentation about prometheus alerts

Signed-off-by: Shadi Abdelfatah <shadi.abdelfatah@gmail.com>

Author: Shadi

content/docs/alerting/configuration.md

@@ -58,10 +58,6 @@ sections.
 
 ```yaml
 global:
-  # ResolveTimeout is the time after which an alert is declared resolved
-  # if it has not been updated.
-  [ resolve_timeout: <duration> | default = 5m ]
-
   # The default SMTP From header field.
   [ smtp_from: <tmpl_string> ]
   # The default SMTP smarthost used for sending emails, including port number.
@@ -98,6 +94,11 @@ global:
   # The default HTTP client configuration
   [ http_config: <http_config> ]
 
+  # ResolveTimeout is the default value used by alertmanager if the alert does
+  # not include EndsAt, after this time passes it can declare the alert as resolved if it has not been updated.
+  # This has no impact on alerts from Prometheus, as they always include EndsAt.
+  [ resolve_timeout: <duration> | default = 5m ]
+
 # Files from which custom notification template definitions are read.
 # The last component may use a wildcard matcher, e.g. 'templates/*.tmpl'.
 templates: