Documentation: fixedTimeZone

The `gitiles.fixedTimeZone` parameter was introduced by 6d920bc73e but
left undocumented.

I (committer) have copied the commit message made by Dave Borowitz with
a bit for formatting. Hence I am assigning him the authorship.

Signed-off-by: Antoine Musso <hashar@free.fr>
Change-Id: I50a57768b33aac84401d04fda27360f4eb7cefaa

Documentation/config.md

diff --git a/Documentation/config.md b/Documentation/config.md
index dc7977d..9cbd132 100644
--- a/Documentation/config.md
+++ b/Documentation/config.md
@@ -7,6 +7,22 @@
 
 ## Core configuration
 
+### Fixed time zone
+
+By default dates are formatted including the local user time zone as that
+matches git tools. Some users/administrators may prefer normalizing to a
+particular timezone so times are directly comparable without doing timezone
+arithmetic.
+
+Setting `fixedTimeZone` to a valid Java TimeZone ID causes all dates in the UI
+to be implicitly converted to this timezone, and the now-redundant timezone
+offset to be dropped from the output.
+
+```
+[gitiles]
+  fixedTimeZone = UTC
+```
+
 ### Cross-Origin Resource Sharing (CORS)
 
 Gitiles sets the `Access-Control-Allow-Origin` header to the