CLEANUP: quic: Code clarifications for QUIC CUBIC (RFC 9438)

CLEANUP: quic: Code clarifications for QUIC CUBIC (RFC 9438)

The first version of our QUIC CUBIC implementation is confusing because relying on
TCP CUBIC linux kernel implementation and with references to RFC 8312 which is
obsoleted by RFC 9438 (August 2023) after our implementation. RFC 8312 is a little
bit hard to understand. RFC 9438 arrived with much more clarifications.

So, RFC 9438 is about "CUBIC for Fast Long-Distance Networks". Our implementation
for QUIC is not very well documented. As it was difficult to reread this
code, this patch adds only some comments at complicated locations and rename
some macros, variables without logic modifications at all.

So, the aim of this patch is to add first some comments and variables/macros renaming
to avoid embedding too much code modifications in the same big patch.

Some code modifications will come to adapt this CUBIC implementation to this new
RFC 9438.

Rename some macros:
  CUBIC_BETA -> CUBIC_BETA_SCALED
  CUBIC_C    -> CUBIC_C_SCALED
  CUBIC_BETA_SCALE_SHIFT -> CUBIC_SCALE_FACTOR_SHIFT (this is the scaling factor
which is used only for CUBIC_BETA_SCALED)
  CUBIC_DIFF_TIME_LIMIT -> CUBIC_TIME_LIMIT

CUBIC_ONE_SCALED was added (scaled value of 1).

These cubic struct members were renamed:
	->tcp_wnd -> ->W_est
	->origin_point -> ->W_target
	->epoch_start -> ->t_epoch
	->remaining_tcp_inc -> remaining_W_est_inc

Local variables to quic_cubic_update() were renamed:
    t -> elapsed_time
    diff ->t
    delta -> W_cubic_t

Add a grahpic curve about the CUBIC Increase function.
Add big copied & pasted RFC 9438 extracts in relation with the 3 different increase
function regions.
Same thing for the fast convergence.
Fix a typo about the reference to QUIC RFC 9002.

Must be backported as far as 2.6 to ease any further modifications to come.