MFC r216168: sh(1): Clean up documentation of built-in commands.

MFC r216168: sh(1): Clean up documentation of built-in commands.

Make sure all built-in commands are in the subsection named such, except
exp, let and wordexp which are deliberately undocumented. The text said only
built-ins that really need to be a built-in were documented there but in
fact almost all of them were already documented.

git-svn-id: svn://svn.freebsd.org/base/stable/8@218343 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f