Clarify documentation regarding search and storage window

The documentation for the feature engagement backend does not mention
in particular the edge cases for how long time back we search or store
data. In particular what 0, 1 and 2+ means for `window` and `storage`.

This CL clarifies this in the documentation, in addition to adding a
comment for the storage part in code.

BUG=None

Change-Id: I69f87e861984e60e61ddf2a2995b78c39d1b77c8
Reviewed-on: https://chromium-review.googlesource.com/773117
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516929}

components/feature_engagement/README.md

@@ -445,11 +445,19 @@ all described below:
 *   `window`
     *   Search for this occurrences of the event within this window.
     *   The value must be given as a number of days.
+    *   For value N, the following holds:
+        *   `0` Nothing should be counted.
+        *   `1` |current_day| should be counted.
+        *   `2+` |current_day| plus |N-1| more days should be counted.
     *   Value client side data type: uint32_t
 *   `storage`
     *   Store client side data related to events for this event minimum this
         long.
     *   The value must be given as a number of days.
+    *   For value N, the following holds:
+        *   `0` Nothing should be stored.
+        *   `1` |current_day| should be stored.
+        *   `2+` |current_day| plus |N-1| more days should be stored.
     *   The value should not exceed 10 years (3650 days).
     *   Value client side data type: uint32_t
     *   Whenever a particular event is used by multiple features, the maximum

components/feature_engagement/internal/feature_config_event_storage_validator.cc

@@ -39,6 +39,9 @@ bool FeatureConfigEventStorageValidator::ShouldKeep(
     return false;
 
   // Too old events should not be kept.
+  // Storage time of N=0:  Nothing should be kept.
+  // Storage time of N=1:  |current_day| should be kept.
+  // Storage time of N=2+: |current_day| plus |N-1| more days should be kept.
   uint32_t longest_storage_time = it->second;
   uint32_t age = current_day - event_day;
   if (longest_storage_time <= age)