string-list.h: move documentation from Documentation/api/ into header

string-list.h: move documentation from Documentation/api/ into header

This mirrors commit 'bdfdaa497 ("strbuf.h: integrate api-strbuf.txt
documentation, 2015-01-16") which did the same for strbuf.h:

* API documentation uses /** */ to set it apart from other comments.

* Function names were stripped from the comments.

* Ordering of the header was adjusted to follow the one from the text
  file.

* Edited some existing comments from string-list.h for consistency.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>