Update documentations accordingly for the design change brought in by PR 214

A design change was brought in by #214: function scripts are not deployed as actual PowerShell functions in memory. When an invocation request comes in, the in-memory functions are invoked instead of invoking the `.ps1` file directly. The purpose of this change is to avoid accessing the `.ps1` file on disk, which is very slow because it's actually the user's storage account mounted on the file system. More information about the change can be found in the PR description of #214.

Documentation needs to be updated to reflect this design change. Also this design change make `$MyInvocation.MyCommand` points to a `FunctionInfo` object instead of a `ExternalScriptInfo` object like before the change. This breaking change should be called out too.