Rename `split_key_docid/1` to `decode_key_docid/1`

Rename the `split_key_docid/1` to `decode_key_docid/1` as it really
is the reverse of `encode_key_docid/2. Also make the parameter names
more symmetric to the encode function.

Change-Id: I26945a36e780a64569505df9f3110d81b5bf9697
Reviewed-on: http://review.couchbase.org/44234
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>