Improve identity template literal tag docs

[jaydenseric]

This PR improves the identity template literal tag docs:

• Tweaked the text from saying tags are “always” used in Apollo projects to “often”, as there are situations where tags are not necessary like when making server side queries via node-fetch that come up in real world projects.
• Replaced the manual copy paste fake tag code with the suggestion to use fake-tag. This utility is easy to install and use and has tests covering Interpolations and escapes. It has been reasonably battle tested.
• Mentioned the /* GraphQL */ comment tag enhancement issue (Support /* GraphQL */ comment tags #224).

See earlier discussion: #224 (comment)

TODO:

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests pass
• Update CHANGELOG.md with your change
• If this was a change that affects the external API, update the README