Fix GN docs for 'gn help deps' and update reference.md.

Fix GN docs for 'gn help deps' and update reference.md.

The help for 'deps' was at best misleading, so we've removed
the misleading parts.

Also, the reference markdown was out of date, so I've regenerated it.

R=brettw@chromium.org
BUG=

Committed: https://crrev.com/aedcc8a8ea65cb485bafbb4452d29c03e404cd5f
Cr-Commit-Position: refs/heads/master@{#350269}

[Dirk Pranke, 2015-09-22 22:57:18]

The CQ bit was checked by dpranke@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-22 22:58:27]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1359113003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1359113003/1

[brettw, 2015-09-22 23:01:20]

lgtm

[commit-bot: I haz the power, 2015-09-22 23:44:55]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-22 23:44:56]

Dry run: This issue passed the CQ dry run.

[Dirk Pranke, 2015-09-22 23:47:57]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-09-22 23:50:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1359113003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1359113003/1

[commit-bot: I haz the power, 2015-09-23 00:06:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-23 00:09:00]

Patchset 1 (id:??) landed as
https://crrev.com/aedcc8a8ea65cb485bafbb4452d29c03e404cd5f
Cr-Commit-Position: refs/heads/master@{#350269}