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
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