Spelling fix in documentation and code comments.

To disabiguate the two homophones, the possessive form
is spelled "its", without an appostrophe. The contraction
"it's" retains the appostrophe. A good way to check which
to write is to try replacing "it's" with the expansion "it is".
If that doesn't make sense, it's the possessive, and should
instead be "its".

I think I got all the "it's" which should be "its". I didn't check all of the reverse, but the code was using "it's" for both pretty consistently.