Document that head() can be a method

[IlyaSemenov]

From here https://ream.js.org/guide/manipulating-head.html it's not obvious that meta tags can be dynamic. Compare with Nuxt documentation: https://nuxtjs.org/api/pages-head

[IlyaSemenov]

Also, it should be documented that tagIDKeyName is set to 'rhid', so that one doesn't need to dig into Ream source code to discover that.