additional documentation for GenBCode
catching up with a fix in master after this PR was submitted

SI-7151 Emit final in bytecode for final inner classes
was fixed in https://github.com/scala/scala/commit/b49b6cf7f0f42d74da854d96eeb0f863357c04cd
more informative name for backend-selection via command-line
GenBCode: Eliminate needless Options

This commit brings GenBCode in line with "Eliminate needless Options"
as promoted by https://github.com/scala/scala/commit/45d61774eb255416c96e983cdb87960ad6415b74