Write kernel-doc comments for queue.h

[jserv]

File list.h follows kernel-doc comments, which allows the generation of API documentation extracted from the comments. However, file queue.h does not, and it would be nice to enforce the kernel-doc formatted documentation for functions.

Expected outcome:

1. Rewrite the comments in file queue.h which follows kernel-doc comments.
2. Simplify the comments in file queue.c: Only one-line summary should be kept as the comments. The programmers should read the generated documentation while modifying queue.c.
3. Move q_release_element from file queue.c to queue.h as a static inline function.