Clarify terminology around directives, shared symtabs, etc.

[toddjonker]

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          Hmm, then I do think we need some clarifying text here--because as it is the dissonance between Ion 1.0 local symbol tables, Ion 1.0/1.1 shared symbol tables (shared modules?), and Ion 1.1 encoding directives are muddled to me, and part of that muddling is the recycling of the shared symbol table schema (which practically makes sense, but really hurts my brain around the mental model)

Originally posted by @almann in https://github.com/amazon-ion/ion-docs/pull/205#discussion_r1177154683