[NaClDocs] Add a snippet about the exitStatus attribute (after crash)

Also add note about calling exit() being unnecessary.

BUG=323855
R=awatson@chromium.org, bsy@google.com

Review URL: https://codereview.chromium.org/108783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241331 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241331