Update source/core/indexes.txt

Added index caveats wrt $or
Small typo under Procedure step 2
merge: pull request with additional information about $or operator
DOCS-1026: correcting typo
Update source/applications/gridfs.txt

Corrected typo: "You also can access information from arbitrate sections of files" become "You also can access information from arbitrary sections of files"
corretion to capitalization
DOCS-1027: clarifying replIndexPrefetch