Leftovers for `docs/syntax.md`

[jaredponn]

In #116, docs/syntax.md was created to specify a LambdaBuffers Frontend file. The chapter is a first draft, and needs improvement.

The remaining work is as follows:

• Rearranging grammar productions to be more consistent with internals: Fixing strange parses #117 (comment)
• Documenting behavior of imports: Fixing strange parses #117 (comment)
• General wording improvements / cohesiveness with other chapters.