[LangRef] Global variable declarations imply minimum size

Adjust the wording added in D78952 to say that global variable
declarations (and interposable definitions) do imply a minimum
size (and alignment) on the global. They just don't imply a
maximum size.

We rely on these semantics in at least two places:

* Global dereferenceability: https://github.com/llvm/llvm-project/blob/2153544865a9733b06579823814c981f735e4201/llvm/lib/IR/Value.cpp#L907
* Global inbounds GEP: https://github.com/llvm/llvm-project/blob/2153544865a9733b06579823814c981f735e4201/llvm/lib/IR/ConstantFold.cpp#L2283

Differential Revision: https://reviews.llvm.org/D143057