Adding cautionary note about manifest_version 2 and Chrome 17.

Adding cautionary note about manifest_version 2 and Chrome 17.

`"manifest_version": 2` doesn't work quite as well in Chrome 17 as we thought it
would. Adding some explicit warnings about its status there: I'd like to merge
these back to the M18 branch, and remove them once Chrome 19 goes out the door.

BUG=116463
TEST=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=125373

[Mike West, 2012-03-06 14:13:08]

Adding a warning to the manifest_version docs regarding the not-so-amazing
compat. with Chrome 17. Staged at
http://www.corp.google.com/~mkwst/no_crawl/docs/manifest.html#manifest_version
and http://www.corp.google.com/~mkwst/no_crawl/docs/manifestVersion.html

[Aaron Boodman, 2012-03-06 22:45:31]

Great, thanks!

LGTM

[commit-bot: I haz the power, 2012-03-07 05:42:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mkwst@chromium.org/9618007/1

[commit-bot: I haz the power, 2012-03-07 05:42:43]

Presubmit check for 9618007-1 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit Messages **
If this change requires manual test instructions to QA team, add
TEST=[instructions].

** Presubmit Warnings **
This change modifies the extension docs but the generated docs have not been
updated properly. See chrome/common/extensions/docs/README.txt for more info.
(Report problems to tessamac@chromium.org.)
 - Changes to chrome/common/extensions/docs/static/manifest.html not reflected
in generated doc.
 - Changes to chrome/common/extensions/docs/static/manifestVersion.html not
reflected in generated doc.
First build DumpRenderTree, then update the docs by running:
  chrome/common/extensions/docs/build/build.py