update clang version

Summary: Update clang version in doc to 4.0.0; add link for clang installation (not trivial to find in macos).

Test Plan: read the doc

Reviewers: schancel, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1021