Remove all mentions of BREW_DIR
Replace Note the by Note that the
Add sentence about virutal environment and decrease warning to note

Also add a link to the virtualenv website so that sysadmins know what virtual
environments are about.
Clarify the Python dependencies installation site-packages paragraph
merge dev_4_4 -D org -I include -E exclude: PR 368 (Cpp: Correct content and markup)
merge dev_4_4 -D org -I include -E exclude: PR 440 (Images tidy dev_4_4)
merge dev_4_4 -D org -I include -E exclude: PR 442 (Homebrew and Python)
merge dev_4_4 -D org -I include -E exclude: PR 443 (Policy images)