Rename "named arguments" to "labeled arguments"

[jchavarri]

In the Function docs, labeled arguments are refered to as "named arguments". This is confusing as OCaml manual refers to them as labeled arguments.

[jchavarri]

JSX page already refers to them as labeled arguments:

reasonml.github.io/docs/jsx.md

Line 13 in 80b2d17

- Props are labeled arguments