Clean up wording to reduce confusion

The description of the timeout parameter was worded misleadingly; it implied that the API would wait until the cluster reached the desired level and then stayed at that level for the timeout. I've tweaked the sentence to remove the risk of confusion.
Added facet support to the percolate api.
Closes #3851
Fix markup
externalize writing "raw" fields to helper method
Merge pull request #3940 from rboulton/patch-1

[Docs] Clean up wording in cluster health api doc
Initial implementation of Snapshot/Restore API