minor: edits to the replication concpts toc page
readability: rs-oplog: very few changes

Before
---
file: build/rs-oplog/json/core/replica-set-oplog.json
source: source/core/replica-set-oplog.txt
stats:
   coleman-liau: 10.714598070739548
   flesch-ease: 48.11670507324044
   flesch-level: 10.840294748124332
   foggy:
      count: 50
      factor: 0.08038585209003216
      threshold: 3
   sentence-count: 36
   sentence-len-avg: 17
   smog-index: 9.861636050157227
   word-count: 622

After
---
file: build/rs-oplog/json/core/replica-set-oplog.json
source: source/core/replica-set-oplog.txt
stats:
   coleman-liau: 10.781154446177844
   flesch-ease: 48.75754639132933
   flesch-level: 10.649214631743167
   foggy:
      count: 49
      factor: 0.07644305772230889
      threshold: 3
   sentence-count: 38
   sentence-len-avg: 16
   smog-index: 9.616208734778699
   word-count: 641

Signed-off-by: Sam Kleinman <samk@10gen.com>
minor edit to oplog document