doc-rst: kernel-doc: fix handling of address_space tags

The RST cpp:function handler is very pedantic: it doesn't allow any
macros like __user on it:

	Documentation/media/kapi/dtv-core.rst:28: WARNING: Error when parsing function declaration.
	If the function has no return type:
	  Error in declarator or parameters and qualifiers
	  Invalid definition: Expecting "(" in parameters_and_qualifiers. [error at 8]
	    ssize_t dvb_ringbuffer_pkt_read_user (struct dvb_ringbuffer * rbuf, size_t idx, int offset, u8 __user * buf, size_t len)
	    --------^
	If the function has a return type:
	  Error in declarator or parameters and qualifiers
	  If pointer to member declarator:
	    Invalid definition: Expected '::' in pointer to member (function). [error at 37]
	      ssize_t dvb_ringbuffer_pkt_read_user (struct dvb_ringbuffer * rbuf, size_t idx, int offset, u8 __user * buf, size_t len)
	      -------------------------------------^
	  If declarator-id:
	    Invalid definition: Expecting "," or ")" in parameters_and_qualifiers, got "*". [error at 102]
	      ssize_t dvb_ringbuffer_pkt_read_user (struct dvb_ringbuffer * rbuf, size_t idx, int offset, u8 __user * buf, size_t len)
	      ------------------------------------------------------------------------------------------------------^

So, we have to remove it from the function prototype.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>