[llvm-readelf/obj][test] - Document what we print in various places for unnamed section symbols.

We have an issue with `ELFDumper<ELFT>::getSymbolSectionName`:
1) It is used deeply for both LLVM/GNU styles and might return LLVM-style only
   values to describe symbols: "Undefined", "Processor Specific", "Absolute", etc.

2) `getSymbolSectionName` is used by `getFullSymbolName` and these special values
   might appear in instead of symbol names in many places.
   This occurs for unnamed section symbols.

It was not noticed because for most cases I've found it is unexpected to have an
unnamed section symbol. This patch documents the existent behavior, adds tests and FIXMEs.

Differential revision: https://reviews.llvm.org/D87763