Added the DistanceUnit.NAUTICALMILES enumeration label with the
corresponding "NM", "nmi", and "nauticalmiles" unit suffixes.

Added the DistanceUnit.NAUTICALMILES enumeration label with the corresponding "NM and "nmi" unit suffixes. Update the docs to match.

Change underscore to blank, as in:

Nautical mile:: 'NM' or 'nmi' or 'nauticalmiles'
Merge branch 'nm-branch' of https://github.com/brian-from-fl/elasticsearch into nm-branch

Conflicts:
	docs/reference/api-conventions.asciidoc