expand background api documentation
expand background api documentation

Add examples and make some text sections more explicit.

BUG=None
TEST=None

Change-Id: I2774057180033093bafcf40a7240167e8e3b4060
Reviewed-on: https://gerrit.chromium.org/gerrit/19748
Reviewed-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

zhang0137/chromite,coreos/chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite