Document that with split-to-hfile data over the MOB threshold will be treated as normal data

documentation update

Update the troubleshooting section of the MOB chapter to include a note for "Why is there data over the MOB threshold in the normal /hbase/data directory rather than in /hbase/mobdir. List the split-to-hfile feature as one source and bulk loading as another. Note that in both cases the next compaction to include those files will write the data out to MOB hfiles.

original

Steps to reproduce this issue.
1. create a table with 1 region, and mob enabled, keep threshold value to 5.
2. Load data into the table, keep the value size should be more than 5.
3. flush the table.
4. observe the mobdir and data dir, hfiles should be there.
5. load data again with different data set, keep the value size is greater than 5.
6. Kill -9 RS where table region is online
7. Start RS

check the mob dir and data dir, both should have 2 hfiles each.
But data dir only have 2 hfiles, that means mob threshold crossed data is considered as normal data.