exportfs: update documentation

exportfs: update documentation

Update documentation to the current state of affairs.  Remove duplicated
method descruptions in exportfs.h and point to Documentation/filesystems/
Exporting instead.  Add a little file header comment in expfs.c describing
what's going on and mentioning Neils and my copyright [1].

Signed-off-by: Christoph Hellwig <[email protected]>
Cc: Neil Brown <[email protected]>
Cc: "J. Bruce Fields" <[email protected]>
Cc: <[email protected]>
Cc: Dave Kleikamp <[email protected]>
Cc: Anton Altaparmakov <[email protected]>
Cc: David Chinner <[email protected]>
Cc: Timothy Shimmin <[email protected]>
Cc: OGAWA Hirofumi <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Jeff Mahoney <[email protected]>
Cc: "Vladimir V. Saveliev" <[email protected]>
Cc: Steven Whitehouse <[email protected]>
Cc: Mark Fasheh <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>