Minor TODO list changes

A few minor typos/notes:

INDEXES

1. On 2nd bullet...
"The main difficulty with this item is the problem of creating an index
that can spam more than one table."

should be span, not spam

2. On 6th bullet
* "Use index to restrict rows returned by multi-key index when used
with non-consecutive keys to reduce heap accesses

For an index on col1,col2,col3, and a WHERE clause of col1 = 5 and col3
= 9, spin though the index checking for col1 and col3 matches, rather
than just col1 "

This is also known as "skip-scanning", so it would be good to use that
phrase in the TODO

3. Whatever happened to the FILLFACTOR todo item? AFAICS it disappeared
sometime at the beginning of August, but I'm not sure why?

REF INTEGRITY

...Didn't we just get rid of deferred triggers?? Perhaps I read that
wrong.

CACHE

1. 1st bullet....prefix it with "Use posix_fadvise() API to let OS know
how files will be used by PostgreSQL. This would allow, for example,
a..." (replacing the word "add").

--
Best Regards, Simon Riggs