regress: Update linear regression docs
regress: Update linear regression docs

JIRA: MADLIB-1460

As part of commit b0991965f7d4007d47902afcf7b319a78d766613 , we updated
num_rows_processed column to be of type BIGINT.  This commit makes the
doc changes for that code change

Authored-by: Ekta Khanna <2fbdcb8e3470ad1f53bbeb24e2deced604b8cfa5@vmware.com>

orhankislal/madlib,orhankislal/madlib,orhankislal/madlib,orhankislal/madlib,orhankislal/madlib