Update the Using the LLVM Toolset document for the 2019.1 release (update no.2, async, with RHEL8 instructions) and minor fixes across docs

see pr in https://issues.jboss.org/browse/RHDEVDOCS-1167

As per rkratky we might release this as an update to 2019.1

AI: rhel8 instructions.
AI: check linked jiras and decide which jiras have to be completed for this update.

AI: cherry pick commits with work from master to 2019.1 branch (thanks, kowen@redhat.com!) -> in PR

AI: see that cherry-picked commits are ok ^

^ PR with cherry-pick/conflict resolved commits, todo, check if info is correct

• versions are correct [x]
• content is correct (2019.3 has wrong info (for llvm). ) [x]

Fixes:

• see linked jiras (might not include all/might not fix everything in the jiras)

Merging changes to 2019.1-master. Publishing docs from 2019.1-master -> 2019.1-stage branches
contact: Tom Stellard