[docs] Reword documentation in terms of SCCs not cycles

Given the example:
header:
  br i1 %c, label %next, label %header
next:
  br i1 %c2, label %exit, label %header

We end up with a loop containing both header and next.  Given that, the describing the loop in terms of cycles is confusing since we have multiple distinct cycles within a single Loop.  Standardize on the SCC to clarify.

Differential Revision: https://reviews.llvm.org/D65299

llvm-svn: 367440