[SPARK-7136][Docs] Spark SQL and DataFrame Guide fix example file and paths

Changes example file for Generic Load/Save Functions to users.parquet rather than people.parquet which doesn't exist unless a later example has already been executed. Also adds filepaths.

Author: Deborah Siegel <deborah.siegel@gmail.com>
Author: DEBORAH SIEGEL <deborahsiegel@d-140-142-0-49.dhcp4.washington.edu>
Author: DEBORAH SIEGEL <deborahsiegel@DEBORAHs-MacBook-Pro.local>
Author: DEBORAH SIEGEL <deborahsiegel@d-69-91-154-197.dhcp4.washington.edu>

Closes #5693 from d3borah/master and squashes the following commits:

4d5e43b [Deborah Siegel] sparkSQL doc change
b15a497 [Deborah Siegel] Revert "sparkSQL doc change"
5a2863c [DEBORAH SIEGEL] Merge remote-tracking branch 'upstream/master'
91972fc [DEBORAH SIEGEL] sparkSQL doc change
f000e59 [DEBORAH SIEGEL] Merge remote-tracking branch 'upstream/master'
db54173 [DEBORAH SIEGEL] fixed aggregateMessages example in graphX doc