NaCl docs: Clarify signal handling and exception handling sections

"Exception handling" is ambiguous, so clarify C++ vs. hardware
exceptions.

"Signal handling" really refers to two features developers might want,
to clarify that.

BUG=none
TEST="make" in native_client_sdk/src/doc

Review URL: https://codereview.chromium.org/77153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236333 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236333