Document ranges and null coalescing

[ColumbusLabs]

Closes #364.

Summary

• add RANGE to the documented types
• document the parser precedence for range and null-coalescing expressions
• explain range endpoints, indexing, string slicing, and current array-slicing limitation
• explain compatible nullable operands and result behavior for ??

Validation

• verified semantics against parser, static checker, interpreter, code generation, tests, and samples
• git diff --check

Bazel is not installed in this environment; the repository contribution guide explicitly exempts documentation-only changes from tests.

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