doc: Add note about the Alertmanager URL path (#4518)

I believe this can often be a source of confusion during initial
configuration because even though the implicit HTTP API prefix set
when using `--alertmanager.external-url` is documented, it is non
obvious behavior.

For example, in #3919.

Signed-off-by: Jack Baldry <[email protected]>

Author: jdbaldry

pkg/ruler/ruler.go

@@ -83,6 +83,8 @@ type Config struct {
 	RulePath string `yaml:"rule_path"`
 
 	// URL of the Alertmanager to send notifications to.
+	// If your are configuring the ruler to send to a Cortex Alertmanager,
+	// ensure this includes any path set in the Alertmanager external URL.
 	AlertmanagerURL string `yaml:"alertmanager_url"`
 	// Whether to use DNS SRV records to discover Alertmanager.
 	AlertmanagerDiscovery bool `yaml:"enable_alertmanager_discovery"`