bug#8678: 24.0.50; `isearch-barrier' referred to in doc string, but it has no doc itself

[Drew Adams]

There are many internal variables in isearch.el that have no doc strings
and no comments that would help communicate the design (intention) for
readers of the code.  I wish this info were added.
 
At the very least, a doc string is needed for `isearch-barrier', because
it is referred to in the doc string of `isearch-fallback'.  IOW, we are
using it to explain something else, yet it is an unknown.
 

In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2011-05-10 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.5) --no-opt --cflags
-Ic:/build/include'