strbuf.h: reorganize api function grouping headers

strbuf.h: reorganize api function grouping headers

The original API doc had something like:

    Functions
    ---------

    * Life cycle

      ... some life-cycle functions ...

    * Related to the contents of the buffer

      ... functions related to contents ....

    etc

This grouping can be hard to read in the comment sources,
given the "*" in the comment lines, and the amount of text
between each section.

Instead, let's make a flat list of groupings, and underline
each as a section header. That makes them stand out, and
eliminates the weird half-phrase of "Related to...". Like:

    Functions related to the contents of the buffer
    -----------------------------------------------

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