doc: split buffer.rst out of api-summary.rst

Buffer heads are no longer a generic filesystem API but an optional
filesystem support library.  Make the documentation structure reflect
that, and include the fine documentation kept in buffer_head.h.  We could
give a better overview of what buffer heads are all about, but my
enthusiasm for documenting it is limited.

[[email protected]: fix kerneldoc warning]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: remove newline at EOF]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Tested-by: Randy Dunlap <[email protected]>
Cc: Pankaj Raghav <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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