Document chrome.bluetoothLowEnergy is available only on Chrome OS.

BUG=460741

Review URL: https://codereview.chromium.org/954053004

Cr-Commit-Position: refs/heads/master@{#318148}

chrome/common/extensions/docs/templates/intros/bluetooth_low_energy.html

+<p class="note">
+<strong>Note: </strong>
+This API is only available on Chrome OS.
+</p>

chrome/common/extensions/docs/templates/public/apps/bluetoothLowEnergy.html

-{{+partials.standard_apps_api api:apis.apps.bluetooth_low_energy/}}
+{{+
+  partials.standard_apps_api
+  intro:intros.bluetooth_low_energy
+  api:apis.apps.bluetooth_low_energy
+/}}