string: Add missing kernel-doc return descriptions

string: Add missing kernel-doc return descriptions

While running kernel-doc validation on linux-next, warnings were emitted
for functions in include/linux/string.h due to missing return value
documentation:

    Warning: include/linux/string.h:375 No description found for return value of 'kbasename'
    Warning: include/linux/string.h:560 No description found for return value of 'strstarts'

This patch adds the missing return value descriptions for both functions
and clears the related kernel-doc warnings.

Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20251118184828.2621595-1-kriish.sharma2006@gmail.com
Signed-off-by: Kees Cook <kees@kernel.org>