docs: document benchmark argument names

[shreejaykurhade]

Summary

Documents how to use ArgName and ArgNames to make generated benchmark names easier to read when benchmarks use numeric arguments.

Fixes #1661

Details

• Adds a Naming Benchmark Arguments subsection to the user guide.
• Shows ArgName for single-argument benchmarks.
• Shows ArgNames for benchmarks with multiple arguments.
• Documents the existing output format, such as BM_SetInsert/size:100/inserts:128.
• Notes that empty argument names leave that value unlabeled.

[shreejaykurhade]

@dmah42 please review

[LebedevRI]

Thank you!