[Documentation] Document the base="true" attribute.

Also fixes up some formatting errors that have snuck into histograms.xml.

BUG=none
TEST=none
[email protected]

Change-Id: If99bf87ea428dbe61204cad9092d3e0ad2c77d7d
Reviewed-on: https://chromium-review.googlesource.com/568854
Reviewed-by: Marijn Kruisselbrink <[email protected]>
Commit-Queue: Ilya Sherman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#486157}
2 files changed