[Docs] Improve cycle and closed path definitions

Improve the cycle definition, by avoiding usage of not yet defined
or only vaguely defined terminology inside definitions.
More precisely, the existing definition defined "outermost cycles",
and then proceeded to use the term "cycles" for further definitions,
which in turn were used to actually define "cycles".

Now, instead only define "cycles". This does not change the meaning
of a cycle, which depends on the chosen surrounding (subgraph) of a CFG.

Also mention the function CFG in the first definition, because later
later definitions require it anyways.

Also slightly improve the definition of a closed path, by explicitly
requiring the inner nodes to be distinct.

Differential Revision: https://reviews.llvm.org/D130891