Clearer message for unused features in chrome://discards/database

The feature columns such as "Updates Favicon In Background" display
either the time when the feature was used, or the time that Site Data
Recorder has been observing it, waiting for use. The latter is
important because the feature state will be kSiteFeatureUsageUnknown
until Site Data Recorder has observed the feature long enough to decide
the site doesn't use it.

Adds an explanatory string to the second case to make it more clear
what this time means in the chrome://discards/database UI.

Change-Id: I2502e381ecca9599528741a3a6f2ba39cb07e221
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6442924
Auto-Submit: Joe Mason <joenotcharles@google.com>
Reviewed-by: Alex Attar <aattar@google.com>
Commit-Queue: Alex Attar <aattar@google.com>
Cr-Commit-Position: refs/heads/main@{#1445015}