Explain unsupported GraphQL features in ReadTheDocs

[pmantica1]

We need to explain that we don't yet support GraphQL features like fragments in ReadTheDocs.

Relevant comment from Bojan:
I think renaming the "Language Specification -> Schema Types" section to "Language Specification -> Syntax" will open up a nice space to talk about this, and also make this section discoverable for its target audience (who would click on a section called "Schema Types"?)