Update source/core/indexes.txt

Added index caveats wrt $or
initial cut of mongoperf doc
text index - clarify examples by using different values; also add Rassi's comments regarding sort and limits
DOCS-1000 mirror changes to extended json wiki
minor: whitespace and cleanup
reorg: replication organization clarity and crud page titling
DOCS-794 update operator reference
merge: DOCS-1000
DOCS-991: amending geospatial index requirement
merge: minor changes to forEach documentation
merge: DOCS-998
DOCS-999 draft reorg landing page
Repeated paragraph
Minor typo
upgrade correction
theme: fixing errant background
minor: fixing internal cross reference
minor: clarifying global lock information
Minor typo
minor: note about dropping text indexes