strbuf.h: drop boilerplate descriptions of strbuf_split_*

strbuf.h: drop boilerplate descriptions of strbuf_split_*

The description of strbuf_split_buf says most of what
needs to be said for all of the split variants that take
strings, raw memory, etc. We have a boilerplate comment
above each that points to the first. This boilerplate
ends up making it harder to read, because it spaces out the
functions, which could otherwise be read as a group.

Let's drop the boilerplate completely, and mention the
variants in the top comment. This is perhaps slightly worse
for a hypothetical system which pulls the documentation for
each function out of the comment immediately preceding it.
But such a system does not yet exist, and anyway, the end
result of extracting the boilerplate comments would not lead
to a very easy-to-read result.  We would do better in the
long run to teach the extraction system about groups of
related functions.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>