Clarify the relationship between byval and readonly/

Clarify the relationship between byval and readonly/
readnone.  Make clearer that readnone functions do not
dereference pointer arguments.  Do not use the highly
ambiguous "side-effects" in the readonly description
(since such functions can have control flow side-effects,
such as throwing an exception, or looping for ever).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57166 91177308-0d34-0410-b5e6-96231b3b80d8