[docs] Revise loop terminology reference.

Motivated by D88183, this seeks to clarify the current loop nomenclature with added illustrations, examples for possibly unexpected situations (infinite loops not part of the "parent" loop, logical loops sharing the same header, ...), and clarification on what other sources may consider a loop. The current document also has multiple errors that are fixed here.

Some selected errors:
 * Loops a defined as strongly-connected components. A component a partition of all nodes, i.e. a subloop can never be a component. That is, the document as it currently is only covers top-level loops, even it also uses the term SCC for subloops.
 * "a block can be the header of two separate loops at the same time" (it is considered a single loop by LoopInfo)
 * "execute before some interesting event happens" (some interesting event is not well-defined)

Reviewed By: baziotis, Whitney

Differential Revision: https://reviews.llvm.org/D88408