small fixup for rand(::BigInt) and update docs

Now the runtime version of GMP is checked against the compile time
version, as suggested by @JeffBezanson. He also noted that the limbs
field of RangeGeneratorBigInt was not specific enough.
Merge pull request #9237 from JuliaLang/rf/rand-bigrange-fixup

small fixup for rand(::BigInt) and update docs