For #13959: improve custom detekt violation output.

Before it used to output the violations all one one line. Now it looks
like:
```
MozillaStrictModeSuppression:
    'import mozilla.components.support.ktx.android.os.resetAfter' at
(17,1) in /StrictModeManager.kt
    Please use `components.strictMode.resetAfter` instead because it has
performance improvements and additional code to monitor for performance
regressions.

MozillaStrictModeSuppression:
    'setThreadPolicy(threadPolicy.build())' at (56,24) in
/StrictModeManager.kt
    Please use `components.strictMode.resetAfter` instead because it has
performance improvements and additional code to monitor for performance
regressions.

MozillaStrictModeSuppression:
    'setVmPolicy(builder.build())' at (71,24) in /StrictModeManager.kt
    NOT YET IMPLEMENTED: please consult the perf team about
implementing`StrictModeManager.resetAfter`: we want to understand the
performance implications of suppressing setVmPolicy before allowing it.
```

mozilla-detekt-rules/src/main/java/org/mozilla/fenix/detektrules/CustomRulesetConsoleReport.kt

@@ -1,13 +1,21 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
 package org.mozilla.fenix.detektrules
 
 import io.gitlab.arturbosch.detekt.api.ConsoleReport
 import io.gitlab.arturbosch.detekt.api.Detektion
 
+/**
+ * A reporter that custom formats violations of our custom lint rules.
+ */
 class CustomRulesetConsoleReport : ConsoleReport() {
-    @Suppress("DEPRECATION")
+    @Suppress("DEPRECATION") // locationAsString
     override fun render(detektion: Detektion): String? {
-        return detektion.findings["mozilla-detekt-rules"]?.fold("") { output, finding ->
-            output + finding.locationAsString + ": " + finding.messageOrDescription()
+        return detektion.findings["mozilla-detekt-rules"]?.fold("") { accumulator, finding ->
+            accumulator + "${finding.id}:\n    ${finding.locationAsString}\n    ${finding.messageOrDescription()}\n\n"
+            // This creates an extra newline at the very end but it's not worth fixing.
         }
     }
 }