strbuf: clarify API boundary

strbuf, as a generic and widely used structure across the codebase,
should be limited as a library to only interact with primitives. Add
documentation so future functions can appropriately be placed. Older
functions that do not follow this boundary should eventually be moved or
refactored.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>