Clarification updates to the Checked C specification.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc#469

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This PR updates the Checked C specification as per the discussions that we have been having about bounds validation when both checked and unchecked pointers (with bounds-safe interfaces) appear in an assignment expression or a function call in an unchecked scope.

Ref: #1158, #1157, #1159 and PR #1150.