Freshening up of the Linux Emulation section, inspired by docs/2780:

• using &rel.current; instead of hardcoding release names
• removed redundant section on determining presence of Linux emulation
• corrected paths
• change 2.2-RELEASE to `2.2-RELEASE and later' in a couple of places
• added a small section on the use of `brandelf'
• removed a old comment about ELF support being new
• made the text a little more precise in a few places

PR: 2780
Reviewed and discussed with: jraynard