docs: Add some struct file refcounting information

Al Viro's recent "braindump" on how struct file refcounting works is too
good to lose, so let's capture it in Documentation/.  There are two changes
here:

 - Clean up the filesystem book, move stuff around, and generally try to
   create some order here.  No text changes other than some connecting
   text.

 - Bring in Al's text, with a fair amount of language tweaking and later
   corrections applied.

Hopefully this is just the beginning.

Jonathan Corbet (2):
  docs: Bring some order to filesystem documentation
  docs: Add struct file refcounting and SCM_RIGHTS mess info

 Documentation/filesystems/api-summary.rst | 150 ++++++++
 Documentation/filesystems/index.rst       | 395 ++--------------------
 Documentation/filesystems/journalling.rst | 184 ++++++++++
 Documentation/filesystems/lifecycles.rst  | 357 +++++++++++++++++++
 Documentation/filesystems/path-lookup.rst |  15 +
 Documentation/filesystems/splice.rst      |  22 ++
 6 files changed, 753 insertions(+), 370 deletions(-)
 create mode 100644 Documentation/filesystems/api-summary.rst
 create mode 100644 Documentation/filesystems/journalling.rst
 create mode 100644 Documentation/filesystems/lifecycles.rst
 create mode 100644 Documentation/filesystems/splice.rst

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2.20.1