doc: add __subsystem attribute and c attributes
doc: add __subsystem attribute and c attributes

Add __subsystem to list of attributes for the parser, and make the c
parser accept the same attributes than the cpp parser, after the change
to use the c parser for ".c" files.

Signed-off-by: Fabio Utzig <382671175562e0df8128054dbdb05a250d79f588@nordicsemi.no>

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