Improve documentation of require() phantomjs differences.

Support for require.paths is no longer a difference, per https://github.com/ariya/phantomjs/issues/11339 . 
And an important difference was not mentioned - PhantomJS has additional search rules.

If phantomjs is run in a directory containing a ./node_modules/mocha/ installation of mocha, these two work:
  phantomjs> require('./node_modules/mocha')
  phantomjs> require('mocha')

The first requires searching from a module directory to a file.
The second requires searching a node_modules hierarchy.
SlimerJS does not do these.

This commit adds the missing differences, and removes the obsolete one.  A mention of require.paths is retained, so there's little information loss.