Merge branch 'jk/strbuf-doc-to-header'

The strbuf API was explained between the API documentation and in
the header file.  Move missing bits to strbuf.h so that programmers
can check only one place for all necessary information.

* jk/strbuf-doc-to-header:
  strbuf.h: group documentation for trim functions
  strbuf.h: drop boilerplate descriptions of strbuf_split_*
  strbuf.h: reorganize api function grouping headers
  strbuf.h: format asciidoc code blocks as 4-space indent
  strbuf.h: drop asciidoc list formatting from API docs
  strbuf.h: unify documentation comments beginnings
  strbuf.h: integrate api-strbuf.txt documentation