Documentation enhancements for 'dll-path', 'hardcode-dll-paths' & FAQ entry

[JBouwer]

Whilst spelunking through the B2 code & documentation - (see discussion #332) - I kept notes to aid my own understanding;

The details 😈 turned out to be a lot more complex than alluded to in the documentation, and henceforth the accompanying pull request.

It is obviously subjective and should definitely be vetted for correctness!

Proposed changes

• doc/src/faq.adoc
• src/tools/features/dll-feature.jam
  Selective enhancements to aid reading & understanding.
  More detail on interplay between these features.
  Removed Mac OSX exclusion remark.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Documentation content changes
• Other (please describe):

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I searched the discussions
• I searched the closed and open issues
• I read the contribution guidelines
• I added myself to the copyright attributions for significant changes
• I checked that tests pass locally with my changes
• I added tests that prove my fix is effective or that my feature works
• I added necessary documentation (if appropriate)