Add information about when OWNERS file paths are and are not expanded.

Follow-up on this CL comment:
https://chromium-review.googlesource.com/c/chromium/src/+/1721083/1/tools/metrics/histograms/extract_histograms.py#279

Bug: 988089, 924136
Change-Id: I1b398d1ce808e511308ae5e3c002a3f70648410d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726596
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682937}

tools/metrics/histograms/README.md

@@ -544,3 +544,15 @@ Please talk with the metrics team if there are more than a thousand possible
 different values that you could emit.
 
 For more information, see [sparse_histograms.h](https://cs.chromium.org/chromium/src/base/metrics/sparse_histogram.h).
+
+# Team Documentation
+
+This section contains useful information for folks on Chrome Metrics.
+
+## Processing histograms.xml
+
+When working with histograms.xml, verify whether you require fully expanded
+OWNERS files. Many scripts in this directory process histograms.xml, and
+sometimes OWNERS file paths are expanded and other times they are not. OWNERS
+paths are expanded when scripts make use of merge_xml's function MergeFiles;
+otherwise, they are not.