tools/gn: sync documentation with reality

tools/gn: sync documentation with reality

Compiler tool definitions use {{source}} and {{output}} to specify the
source and output files, not $in and $out.

Also fix some typos and spurious commas.

Committed: https://crrev.com/36d057d7f2eb9e97c693e9b241f9880212768e7e
Cr-Commit-Position: refs/heads/master@{#321605}

[mdempsky, 2015-03-19 23:17:14]

mdempsky@chromium.org changed reviewers:
+ brettw@chromium.org

[mdempsky, 2015-03-20 18:11:28]

mdempsky@chromium.org changed reviewers:
+ dpranke@chromium.org

[brettw, 2015-03-20 18:14:13]

lgtm

[mdempsky, 2015-03-20 18:16:00]

The CQ bit was checked by mdempsky@chromium.org

[commit-bot: I haz the power, 2015-03-20 18:16:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1020073002/20001

[commit-bot: I haz the power, 2015-03-20 18:41:29]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-20 18:46:17]

Patchset 2 (id:??) landed as
https://crrev.com/36d057d7f2eb9e97c693e9b241f9880212768e7e
Cr-Commit-Position: refs/heads/master@{#321605}