Problem:
`:help dev-name-common` states that "buf" should be used instead of
"buffer" but there are cases where buffer is mentioned in the lua API.
Solution:
- Rename occurrences of "buffer" to "buf" for consistency with the
documentation.
- Support (but deprecate) "buffer" for backwards compatibility.
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>
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