Fix telemetry docs (#794)

* Replace warning level with warn.

* Update help for telemetry level filters

* Update URL for directives

* Use a link variable

* Open link in new tab

I chose not to move the title `[]` bit to the url definition so that it still reads well as text

Author: NickLarsenNZ

modules/concepts/pages/observability/telemetry.adoc

@@ -1,4 +1,5 @@
 = Telemetry signals in operators
+:env-filter-syntax: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html#example-syntax
 
 [NOTE]
 ====
@@ -46,9 +47,9 @@ telemetry:
     format: null  # <3>
 ----
 
-<1> Boolean: `true, false`
-<2> Enum: `error, warning, info, debug, trace`
-<3> Enum: `plain, json`
+<1> Boolean: `true`, `false`
+<2> String: `error`, `warn`, `info`, `debug`, `trace`, `off` (or {env-filter-syntax}[more complex filters,window=_blank])
+<3> Enum: `plain`, `json`
 
 == File logs
 
@@ -65,9 +66,9 @@ telemetry:
     maxFiles: 6            # <4>
 ----
 
-<1> Boolean: `true, false`
-<2> Enum: `error, warning, info, debug, trace`
-<3> Enum: `never, daily, hourly, minutely`
+<1> Boolean: `true`, `false`
+<2> String: `error`, `warn`, `info`, `debug`, `trace`, `off` (or {env-filter-syntax}[more complex filters,window=_blank])
+<3> Enum: `never`, `daily`, `hourly`, `minutely`
 <4> Unsigned Integer
 
 == OpenTelemetry logs and traces
@@ -88,6 +89,6 @@ telemetry:
     endpoint: null # <3>
 ----
 
-<1> Boolean: `true, false`
-<2> Enum: `error, warning, info, debug, trace`
-<3> String: E.g. `my-collector:4317`
+<1> Boolean: `true`, `false`
+<2> String: `error`, `warn`, `info`, `debug`, `trace`, `off` (or {env-filter-syntax}[more complex filters,window=_blank])
+<3> String: E.g. `https://my-collector:4317` (Note: it must contain the scheme)