ORA-237: Number in "Available for Grading" column drops rapidly

Added the following under "Grade an Open Response Assessment Problem":

After you've graded enough responses for AI assessments to start, the
number of responses in the Available to grade column decreases rapidly
as the algorithm grades responses and returns them to your students in
just a few seconds. No student data is lost.

Also corrected a few minor formatting problems.
Merge pull request #1908 from edx/sylvia/documentation/ora-237

ORA-237: Number in "Available for Grading" column drops rapidly