Help: target_include_directories does not allow target names

Since commit v2.8.11~227^2~1 (Don't allow targets args in the new target
commands, 2013-01-29) the target_include_directories command does not
support target names on the right hand side, but that commit forgot to
remove it from the docs.  It was never released with such support.
Update the command documentation now.
Merge topic 'doc-target_include_directories-fixup'

1d22ee67 Help: target_include_directories does not allow target names