Updating jumbo documentation.

The default chunk size is not 200 anymore, for a number of
months.

Change-Id: I8370e472628ddcf1c49bdc09d989bdadf3ea78b3
Reviewed-on: https://chromium-review.googlesource.com/946097
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Jens Widell <jl@opera.com>
Cr-Commit-Position: refs/heads/master@{#540517}

docs/jumbo.md

@@ -51,10 +51,10 @@ source files.
 
 ## Tuning
 
-By default at most `200` files are merged at a time. The more files
-are merged, the less total CPU time is needed, but parallelism is
-reduced. This can be changed by setting `jumbo_file_merge_limit` to
-something else than `200`.
+By default at most `50`, or `8` when using goma, files are merged at a
+time. The more files that are are merged, the less total CPU time is
+needed, but parallelism is reduced. This number can be changed by
+setting `jumbo_file_merge_limit`.
 
 ## Naming