of: linux/of.h: fix kernel-doc warnings

Correct kernel-doc notation warnings:

../include/linux/of.h:1211: warning: Function parameter or member 'output' not described in 'of_property_read_string_index'
../include/linux/of.h:1211: warning: Excess function parameter 'out_string' description in 'of_property_read_string_index'
../include/linux/of.h:1477: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Overlay support

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210417061244.2262-1-rdunlap@infradead.org