kiji-express: DOCS-154. Add documentation about row skipping behavior in KijiExpress.

issue: https://jira.kiji.org/browse/DOCS-154
review: https://review.kiji.org/r/1628
commit: https://github.com/kijiproject/kiji-express/commit/d43fe2c2e6b1aaf8286f201d5a5afe07e3e85da5
kijiproject.github.com: DOCS-154. Add documentation about row skipping behavior in KijiExpress.

issue: https://jira.kiji.org/browse/DOCS-154
review: https://review.kiji.org/r/1627
commit: https://github.com/kijiproject/kijiproject.github.com/commit/af0f2f98beff853366fab3d6a0eee4b5810f409a