Update Questions 15.10 - 15.20:

• Change from AENXXXXX-style references to explicit identifiers added to the Handbook previously [1]
• Make markup of tip(1) consistent

[1] http://lists.freebsd.org/pipermail/cvs-doc/2008-August/018741.html

Reviewed by: trhodes, remko, keramida
Approved by: gabor (mentor)