Document the units of -ddump-timings

Right now, in the output of -ddump-timings to a file, you can't tell what the units are:

```
CodeGen [TemplateTestImports]: alloc=22454880 time=14.597
```

I believe bytes/milliseconds are the correct units, but confirmation would be appreciated. I'm basing it off of this snippet from `withTiming'`:

```
when (verbosity dflags >= 2 && prtimings == PrintTimings)
  $ liftIO $ logInfo dflags (defaultUserStyle dflags)
      (text "!!!" <+> what <> colon <+> text "finished in"
       <+> doublePrec 2 time
       <+> text "milliseconds"
       <> comma
       <+> text "allocated"
       <+> doublePrec 3 (realToFrac alloc / 1024 / 1024)
       <+> text "megabytes")
```

which implies time is in milliseconds, and allocations in bytes (which divided by 1024 would be KB, and again would be MB)
24 jobs for master in 778 minutes and 30 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Lint
passed #283529
lint
lint-linters

00:00:13

passed #283532
lint
lint-submods-branch

00:00:16

passed #283530
lint
lint-testsuite

00:00:15

passed #283531
lint
typecheck-testsuite

00:00:14

 
  Quick Build
passed #283533
x86_64-linux
hadrian-ghc-in-ghci

00:08:31

 
  Build
passed #283534
x86_64-linux
validate-x86_64-linux-deb9-hadrian

02:59:25

 
  Full Build
failed #283537
aarch64-linux allowed to fail
validate-aarch64-linux-deb9

03:15:02

passed #283538
armv7-linux
validate-armv7-linux-deb9

04:24:39

passed #283539
x86_64-linux
validate-i386-linux-deb9

01:11:17

failed #283546
new-x86_64-windows test allowed to fail
validate-i386-windows

passed #283536
x86_64-darwin
validate-x86_64-darwin

01:10:41

passed #283540
x86_64-linux
validate-x86_64-linux-deb9-debug

02:39:34

passed #283542
x86_64-linux
validate-x86_64-linux-deb9-dwarf

02:27:35

passed #283541
x86_64-linux
validate-x86_64-linux-deb9-integer-simple

01:40:27

passed #283535
x86_64-linux
validate-x86_64-linux-deb9-unreg-hadrian

04:45:42

passed #283543
x86_64-linux
validate-x86_64-linux-fedora27

01:41:23

failed #283545
new-x86_64-windows test allowed to fail
validate-x86_64-windows

03:11:06

failed #283544
new-x86_64-windows test allowed to fail
validate-x86_64-windows-hadrian

01:00:19

 
  Cleanup
passed #283547
x86_64-darwin
cleanup-darwin

00:00:27

 
  Packaging
passed #283548
x86_64-linux
doc-tarball

00:24:01

 
  Testing
manual #283549
x86_64-linux allowed to fail manual
hackage
passed #283550
x86_64-linux
perf-nofib

00:26:09

 
  Deploy
passed #283551
x86_64-linux
pages

00:02:12

failed pages:deploy

00:00:00

 
Name Stage Failure
failed
validate-aarch64-linux-deb9 Full Build
Uploading artifacts...
ghc-aarch64-linux-deb9.tar.xz: found 1 matching files
junit.xml: found 1 matching files

performance-metrics.tsv: found 1 matching files

Uploading artifacts to coordinator... ok
id=283537 responseStatus=201 Created token=YRhAdbv8
Uploading artifacts...
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=283537 responseStatus=201 Created token=YRhAdbv8
ERROR: Job failed: exit code 1
failed
validate-i386-windows Full Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job log
failed
validate-x86_64-windows-hadrian Full Build
Git branch:   12-6-stable
GO version: go1.13.4
Built: 2019-12-22T11:55:34+0000
OS/Arch: windows/amd64
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=1144 revision=ac8e767a version=12.6.0
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 1
failed
validate-x86_64-windows Full Build
GO version:   go1.13.4
Built: 2019-12-22T11:55:34+0000
OS/Arch: windows/amd64
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=7560 revision=ac8e767a version=12.6.0
junit.xml: found 1 matching files

WARNING: Failed to load system CertPool: crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=283545 responseStatus=201 Created token=SQQvzaze
ERROR: Job failed: exit status 1