src: doc: Major re-work of user packet buffer documentation

Divide functions into a hierarchy:
 top-level: Functions all programs that modify data will use
 2nd-level: Rarely-used functions
 3rd-level: Functions not to use (should have been declared static)
Only the top-level functions appear on the "User-space network packet buffer"
page, which looks a lot less daunting than it used to.

Parameter descriptions all match prototypes

All non-void functions have a "Returns" paragraph

Code change:

 pktb_alloc: set errno to EPROTONOSUPPORT before doing error return because
             protocol is not supported

Detailed other updates (top-level)

 pktb_alloc: - Add "Errors" para
             - Add "See also" para

 pktb_data, pktb_len: Add "appropriate use" line

 pktb_mangle: Add warning to use a different function unless mangling MAC hddr

 pktb_mangled: Add usage hint line

Detailed other updates (2nd-level)

 pktb_mac_header: Point out only for AF_BRIDGE

 pktb_tailroom: Point out no dynamic expansion

 pktb_transport_header: Add note that programmer must code to set this

Signed-off-by: Duncan Roe <duncan_roe@optusnet.com.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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