Add multiple source file and templating support to file.append (#6905)

Allow states/file.append() to use multiple source files and templating
to generate the text to be appended to the 'named' file.
Update Docstring
Remove the warning about bug #7290 now its fixed.
Merge pull request #7296 from mech422/add-file-append-template-support

Add multiple source file and templating support to file.append (#6905)