Merge branch 'nfs' into docs-next

Daniel W. S. Almeida writes:

  This series converts a few docs in Documentation/filesystems/nfs to RST.
  The docs were also moved into admin-guide because they contain
  information that might be useful for system administrators

  Most changes are related to aesthetics and presentation, i.e. the content
  itself remains mostly untouched. The use of markup was limited in order
  not to negatively impact the plain-text reading experience.