doc: module: revert misconversions for MODULE_IMPORT_NS()

This reverts the misconversions introduced by commit cdd30ebb
("module: Convert symbol namespace to string literal").

The affected descriptions refer to MODULE_IMPORT_NS() tags in general,
rather than suggesting the use of the empty string ("") as the
namespace.

Fixes: cdd30ebb ("module: Convert symbol namespace to string literal")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>